I think that the buzzy 'up' feeling you are supposed to get after exercise is a complete myth. I bet only people who are really fit in the first place get it. I exercise regularly and your description about being hit over the head with a tree is an excellent description of how I feel after. I had to give up taking lunchtime classes because it just wiped me out for the entire afternoon.
Sorry for random commenting from an habitual lurker but that hit by a tree thing really rang a bell for me.