Hardest part of running is not running too fast, even when you can. It's a recipe for injury. Instead, let your body grow into running, which initially takes a couple of years to grow your tendons and bones, which is not really finished, ever: you can always improve. I only learned to take it easy after 40, only then having the peace of mind of just enjoying running, instead of wanting to be at the next crossing, the next bridge or the next footpath. Just to enjoy the running that you do and reach all the landmarks while just running along. Doing a race once in a while will help to keep the training on a lower intensity, but it will always remain difficult not to run too fast. Injuries will be just around the corner if you're not careful.