Just remember, when you are doing sit-ups, someone else is practicing. I say the best remedy for endurance is to play, play, play. When I find my endurance failing, I go to the Arban and play a bunch of the little phrasing melodies starting on page 191. I try and see how many I can play in a row (beautifully!) Once my tone gets rough I stop and rest and go back to them later. The Clarke studies are very good as well as well as the Concone etudes.