This is also a big issue for me. I feel like I have spent my whole life trying to get fit, and it is like pushing the boulder up the hill...
This morning before my muesli I drank an organic celery and carrot juice with wheatgrass. Felt it pulsing through me, off to the pool for a swimming class, which went pretty well. Came home and could feel energy starting to dip, had another juice but didn't feel any better. Energy started to plummet, needed to eat quickly, had a fry up of vege sausages, baked beans and scrambled eggs (yes, I am in England!) - not very healthy but saved me from feeling like I was going to crash. Energy still low after this. Now drinking barley coffee substitute and waiting for energy to come back so I can pursue work etc.
Happens to me almost every time I swim or pursue any vigorous activity. What can we do, apart from persevere?