I definitely experienced this in training for my marathon and half Ironman (which I had stupidly signed up for in consecutive months). I didn't follow my training plan properly and tried to cram all the volume in the last month or two and it didn't end well. I was able to make it through the half Ironman through pure adrenaline, but I laid in bed for the following two weeks and barely moved. I had no motivation to do anything whatsoever.

It turns out that I was deficient in plenty of vitamins: D, B12, iron. I'm sure these were all symptoms of OTS. I didn't feel the same in any of my workouts until maybe 4 months later. Even then, it took very gradual build-up after those 4 months to get back to where I was. Taking vitamin supplements helped, sure, but the true cure was just ramping down the exercise a lot and getting proper sleep over months.

I empathize with these athletes because exercise is addicting; it feels almost morally wrong to take days off. There's also always someone better than you. But as I got older, I realized that recovery is just as important (if not, more important) than the actual work. In fact, that's where all the gains are made.

I found a few helpful pointers with structuring my workouts now is to:

* Increase training volume <10% W/W. It gives your body time to adapt and also helps with preventing injury.

* Mixing in recovery weeks is also really helpful with helping your body adapt.

* There's no way around doing a lot of volume when training for endurance events, but keeping your HR lower the whole time is a good way to make sure you don't overtrain and can show up the next day.