She recommends if you're a beginner runner, long runs should just be easy. Not speed.
For more experienced runners, the fast effort should be in the middle of the run. Before you get fatigued and your form starts slipping. After you've run enough to have fully warmed up.
four to six miles at goal marathon pace in the middle of, say, a 16-miler, ideally staying within three to five seconds of goal pace.
Helping you learn to lock in your goal pace.