I've never done a long-term fast but I can tell you that ever since I abandoned consciously eating breakfast, lunch and dinner I have tremendously boosted my energy and mood levels. My style of eating I suppose mirrors Martin Berkhan's Leangains protocol, where every day consists of a 16 hour fast followed by an 8 hour window of feeding, although I was doing it long before I heard about Leangains.
The way this works for me is I eat lunch around noon, then graze / snack intermittently until dinner around 7 or 8. Then I'll just have water until bedtime. When I wake up in the morning I'll have tea with a dollop of cream or coffee with a spoonful of sugar but that's all until lunch again. Since I strength train and I'm in a size phase now, I have to get in all my calories in that window, but I'm still planning on a 36-hour fast later this month.
1) eat real food - more vegetables, moderate meat, moderate fruits, less grains, less sugar, less vegetable oils.
2) exercise - moderate intensity cardio, sprinting, heavy lifting, dedicated stretching and mobility.
3) live - relax, de-stress, meditate.
Disclaimer: I'm not professionally qualified to make any formal recommendations. I've just done my homework and I'm my own guinea pig. All of my data, unless otherwise cited, comes from a sample size of n=1 (me).