What point is considered exhaustion, and how often should this be performed? I've gone on 20+ mile bike rides that have left me pretty tired, but I don't think I felt any better mentally/slept any better. Is it more of an "over time" thing?

I've read advice like this a lot, but it seems like such a difficult thing to jump into, especially if I've not had much success with small trials (i.e. the 20 mile bike rides). It also makes me wonder what the intended end goal is; if I spend all of my time exercising, and that does end up fixing me, what do I go back to?