2-4x/wk, I exercise from about 5:30--7pm (1 hr of weights + 4mi treadmill at 7:30-8:00 pace). I'd say I work out from a 5 to 8 on a scale of 1-10, with 10 being highest intensity. I then shower, dine...and afterwards typically feel rather tired and low-energy until bedtime, such that I am fairly ineffective for much beyond just kind of taking it easy. Sometimes this occurs a day or so after a workout. I'd much prefer to have more energy, but also wish to have reasonable fitness.
The thing I've found that is most consistently correlated with retaining energy post-workout is my fitness level, unsurprisingly. If I haven't been working out in months and try this, it is like a truck hit me. If I am in good shape, it's not as severe. I do try to do what I can in terms of eating, drinking, and sleeping properly. My diet is largely vegetarian, fairly health conscious, and I drink sufficient water. I could probably sleep better, and am working on that (that might be a big component).
My questions are: Do I simply have to accept that I'll have this sort of energy drop, or is there a way to mitigate it significantly? If so, how? How much should one's age matter?