I used to workout in the gym as running on the runner until my knee got hurt due to tennis. Then I switch to the sking machine (don't know the term, the treadmill without the handheld stick) and my knee feel a lot better However, I feel like going to hell after 30 min on the thing. I wonder if it can build up the stamina better For swimming, it's alot easier for me where I can swimming for 90 min non stop and didn't feel too exhausted when finished Does the gym work and swimming serve the same purpose for better stamina? Or swimming won't work unless I feel like dying out there? which is a more important element? Intensity or duration?