not sure how old you are and what kind of endurance you are talking about. i mean. do you mean sitting for long periods of time and then suddenly standing? or going long periods of time without eating? or eating erratically. or exercising erratically? and then trying to be energtic all the time?
i think where older people go wrong is that we feel guilty if we are tired. if you are tired, don't fight it. go home and sit on the front porch. get your harmonica out.
food seems to be the next item of 'energy' giver. not sugar! that's the problem with our first diet learned. soda, candy, ice-cream. these are not energy foods. they boost it for the moment, but cause energy lows in the late afternoon/evening. a fifteen to twenty minute nap isn't worth fighting. take it. eat something afterwards. go exercise. and then, don't practice too much except on weekends. that way - you'll still have some energy left to live ther est of your life! (oh, and don't forget to do some gardening in the backyard - it's a real destressor).
Topic: painkillers (legal of course)