A lot of the exhaustion you feel after 30 minutes is probably NOT from being out of shape, but from doing something new. It takes a lot of mental effort when you're trying a new activity or on a new bike or even a new route, and you don't know how to expend your physical effort efficiently, so you wear out faster than you normally would. I'd expect that sort of "newness" exhaustion to go away very quickly, and then you can focus on really physically wearing yourself out!

Sarah