the idea of training endurance is that you have more energy and can train more. Endurance training cost scales with lvl of endurance, that's make sense. What doesnt make sense is that for some reason Strenght training cost also scales with endurance lvl, making it kinda pointless and weird. With more endurance training becomes less and less effective, it costs more energy and gives same experience. You have new fresh amazonian training on lvl 8, but it is final lvl up so it is kida useless, you use it few times, lvl up to lvl 9 and the game ends here for now.
Another annoying thing is that training endurance sends you to training strength window, making it harder to train endurance in the first play, so you need train endurance, and go back to gym menu instead of just pressing one button again.
(i kinda like the rivalship, but it would be more interesting if you rival would grow from days and not your lvl, and also it would be better if we could surpass her on some stages so she would be more motivated to reach new stages sooner. Now it kida feels like one way play. We train, but she is always a little ahead)