I'm in the military, on a SOF team, and my physical activity level is usually extremely high. I spent over two years in training, and a good part of it was simply taking beat-downs (constant, intense physical activity to see who might quit). For months at a time, we'd usually put in 8-12 hours a day of almost constant putting out. The regimens included just about everything you could think of... weight training, swimming sprints with bricks and a full uniform, ocean swims, running 10-15 miles a day with a ruck, calisthenics, etc.
After a while, you just get used to it and you don't get very sore or tired anymore and you learn to just tune it all out. I never really got very sick during training. To do so would have been devastating most times. An ear infection, a cold, or a day or two long viral thing might pass by occasionally, but that was about it.
Now that I'm done with training, I still maintain a high training/activity level. I utilize back-off weeks and recovery nutrition/workouts whenever possible and applicable.
The problem I have had is that when I take leave and go on vacation, and I can eat and sleep all I want, I crash. I'll be okay the first two or three days, but after that, suddenly I'm sleeping 12 hours a day and have absolutely no energy. My immune system will drop and I'll get sick. Almost every time I've taken more than a week of leave in my career, I've come back from all that "rest" feeling more run-down than before and with mono, some virus with similiar symptoms, a severe ear/sinus infection... whatever.
I have read that to fully recover from some types of training can take as long as the training itself, and I've heard (anecdotally) that some of the training we go through can take months or years to fully recover from.
Does anyone else have similiar experiences when coming off a tough training regimen? Any ideas on what to do about it?