Whenever I've had a really tough workout, I usually start to feel great within about half an hour - mainly because of the endorphins rushing through my body.
The inverse can also be true when I do something that isn't great for my body, like eating a greasy burger and chips. It feels highly comfortable at the time but doesn't sit so well half-an-hour later. Alcohol is the exception to the rule, where I feel terrific while drinking and half-an-hour later - becoming much better looking and charming - before struggling about 12 hours post-event!
This is how it works: Short-term boosts often don't serve us in the long run. Ask anyone who's achieved something great in their lives, and they'll tell you that it wasn't easy at the time. They've had to push themselves to get there.
I know when I'm 80-years-old, I'll look back over my life and I'll see that everything I'm really proud of was achieved surrounded by a period of uncertainty because it pushed me to, or close to, my capacity. By pushing myself, I've learned another important lesson: the more you do something and expose yourself to that uneasiness, the more comfortable you become being uncomfortable.