I do this 4 or 5 times a year and have been for 5 years now. Then when it's done, stay on a decent diet especially No Fast-food.
I'm about to turn 61 and am in good shape but it's work. I'm not doing anything crazy like triathlons but every day in the gym and or exercise like a 5 mile walk.
If you want your energy up? Exercise and eat decent but NO FAST FOOD....PERIOD.