As others have said, there's always an adjustment period, and before you know it you'll be doing it without even thinking about it.

When I started at my current workplace five years ago, I was out of shape after spending almost ten years either at uni, or in a succession of office jobs. So going back on my tools as a welder was a massive shock that I wasn't really expecting. But suddenly, six months or so in, I realised that I was finding the work much less physically demanding, which felt really gratifying. I wasn't fit by any means (I never have been), but fit enough to do the work.

Then, 18 months ago I got promoted to a desk job, and I'm fat as shit again. I'd be fucked after a day back on my tools.

There is always an adjustment period. I find after am extended time off from physical labor it takes me about 3 weeks to get fully used to it again. As you work more and more, your muscle tone will return and your cardio performance will improve.

STRETCH STRETCH STRETCH! you might think you stretch but you definitely aren't doing enough. and it does wonders for keeping your muscles working. Stretch before you start. Stretch during breaks. Stretch when you quit and when you get home too. Yes it looks goofy but IT WORKS. Keep that shit loose before it gets sore and cramped. Look up YouTube videos on proper stretches and focus on legs and back.

HYDRATE! You should be sucking water from a bottle constantly when sweating in a hot physical labor environment. You need to drink probably 3x or more your "normal" water intake, you should be peeing only light yellow on every break. You get dehydrated and you'll feel like shit.

Seconding the recommendation to S T R E C H and hydrate yourself, aim for minimum every couple hours. This will help keep your body in shape for longer no matter whether you work at a desk, standing, walking around a warehouse, a kitchen, a school, jobsite, WFH or whatever you do.

How's your hydration? Poor hydration can lead to muscle cramping, among many other issues.

A lot of great advice in this thread.

Adding a note here about socks. Don't bother with Bombas. They are fine, but no better than other generic socks you will find out there. I was all in to the hype of them too but after using them for a few years i found they don't hold up any better than other socks. I wouldn't consider them an "investment".

Now, if you want to invest in a sock, consider getting Darn Tough. They are more expensive, however their wool and synthetic socks are pretty solid. I find they hold up way better than my Bombas ever did, are much more comfortable, moisture wicking of wool is very good and, best part, if they fall apart, wear down and get holes, Darn Tough will replace them. They will have you send them back and give you a credit to get a new pair. Full credit. It's fantastic. Highly recommend.

+1 for all the advice here, with one extra one: Make sure you're getting enough protein in your diet. Your muscles are getting torn up, and that's good - that'll make them grow back stronger! But only if you're getting enough protein to build them back. I boil up 6 of these guys at a time and pop two a day for breakfast for just a little extra protein: https://food52.com/recipes/35930-momofuku-s-soy-sauce-eggs

Stretch and do some light exercise right before you leave for work just to get the blood flowing. Take regular breaks just to stretch as well, and make sure to use as much of your breaks as possible off your feet. I've worked a few of these jobs and the worst thing you can do to yourself is ignore your breaks or even taking your breaks but not sitting down.

Make sure to get good sleep before your shifts and go to bed at a reasonable time. At the risk of accidentally sounding pro-capitalist here when I'm not trying, human bodies are actually uncommonly great at exercising and being on our feet for extended periods of time, so you should get used to it eventually. The first few weeks are definitely not going to be fun though.

Placing orthotics bellow your arches is very harmful in the long term. In general you shouldn't prevent your foot doing what it is designed to do (big heel drops, fat soft shoe soles, orthotics) unless there is a problem (read injury) and only temporarily (until you recover). So are narrow toe-boxes in shoes, your toes should be able to move freely and naturally. If they can't, the restriction will create irreversible (read: even surgery won't completely fix what they cause) problems, that mess up all the bio-mechanics of the leg. I wish I knew this when I was younger, working 8-10-12 hour shifts (yeah, I know), as a waiter/barman.

Btw, it might sound counter-intuitive, but proper running, relaxed and a little each day (even as little as 10 minutes) can help getting your legs stronger, relieve stress, restore fascia (without stretching, static stretching never ever worked well for me) and keep it flexible and strong, reset nervous system firing patterns on your shoulders (moving your hands like you do in running with the proper form is way more effective than PT exercises like trap-3-raises for the traps) to counter balance the amount of time you spend looking down, help re-align your spine, and pretty much invigorate your whole body.

But most importantly, rest and eat well. This will be the defining factor on whether your body will adapt and get stronger or not, and how long it will need to do it. We are supposed to be standing all day (not facing downwards though), your feet shouldn't be the issue here, your neck & shoulders are the part that is assuming the unnatural positions for extended periods of time, so as often as you can break them and do some gentle full range of motion movements (a.k.a dynamic stretching) the better.

Regardless, good luck with your new job! :-)

I've been working in kitchens for about ten years. I was pretty much sedentary before I started too, and yeah, the first few weeks are the hardest, but you do get used to it. NGL it's hard work though. It's rough on your body, mind and soul. Good on you for taking steps now to look after yourself, I wish I had when I first started. Aside from @bermuda@beehaw.org's advice, here's some tips from an old line cook:

If there are rubber mats in your kitchen (this is OHS in some places), stay on them as much as possible. Aside from the non-slip aspect, the extra bit of cushioning is easier on your knees/ankles than tiles/concrete.
Other usual stuff like proper lifting technique.
If you're standing at a bench or grill for long periods try not to hunch over too much, it's easy to do if you're concentrating on what you're doing.
Alternate your posture by either bending your knees a little or spreading your legs a bit. Try not to stand flat footed for too long, and try to avoid keeping all your weight on one leg for long periods.
A good sharp knife that fits your hand along with proper cutting technique will save your wrists/shoulders from a lot of strain.

sleep with wrist guards on. the kind with the metal plates in em, that keep your wrist tilted back a bit. both hands. uncomfortable at first but easily the best long term investment for your wrists you can make (this applies for your thumb too, it's all connected)

the fact your thumb is going numb is pretty worrisome. if you can, baby it in the shower hot water and massage it. or alternate running hot and cold water on it (4 min hot, 1 min cold). stretch it, your wrists, arms and shoulders (it's all connected). look up nerve glides and do those as well

i went to wrist pt for a while, this is all stuff i learned from it. hope it helps, seems like you're covered on feet/leg stuff

Get a remote customer service position if possible. Better pay and they usually ship you the equipment.

I dont know how old you are OP or in what good state of health you are, but assuming youre young and ablebodied it should get easier.

I worked fast food for years and while it was emotionally and physically draining to do more than 40 hours a week it became mostly second nature after a while.

I made friends and spent time with good people in my fastfood, including finding my future college roommate which admittedly helped change the trajectory of my life(I may not have gone to the uni I went to if I had to go alone). In a lot of ways I was the perfect age for it because a lot of my coworkers were people close to or around my age and it made the experience a bit more fun and the hard work of it all was a bonding experience that brought us together. Future jobs I would take didnt have that.

That said, get out as soon as you can. It's easy to let the fuzzy rose colored filter of time make it out to be good, but it was work. Long nights, long shifts, lots of angry mean customers, stupid workplace drama from people fighting for what little crumbs we got paid as shift and team leaders. It's unjust but I find that the more money that I make in progressively newer jobs the less I have to work. Believe me I understand leaving is easier said than done, and you only just started, but if you can get better reach for it and grab it.

I say this not to preach some bootstraps nonsense, but because fast food workers are underestimated. Good ones have to have strong customer service skills, thick skin, an ability to quickly learn and multitask, and the ability to quickly make and prep the food. I also mention this because it's easy, especially when youre tired from working your fast food hours, to get stuck. I myself worked that fast food for close to 5 years and I know others who are still working there all these years later.

I am in a similar position in my job. I am more of a farmhand, so I do a little bit of everything. I just cope with energy drinks lol. Maybe I should get some compression socks.