I've been exercising regularly for too long to have a good idea of what not doing so would do to my energy levels. When I stop for a while, it's usually because I'm on vacation or too busy to have the time, and either way I feel the resulting diffs would be worthless; being in new or unusual situations (as I generally am on vacation) consistently boosts my energy, and being busy enough that I don't have six to ten hours a week or so to spare predictably drains it.

Within my usual range of variation, I've noticed effects on mood, but not on energy or willpower.