One other point on the quest for cardio-- this also was the reason I wanted to get to a larger wheel. I kept seeing videos of people buzzing around on 20-24" unicycles effortlessly…
However, I’m about 1.5 months into riding, and I can mount and ride the 24 for as long as my heart rate can stand it-- which is only about 200’. Then I have to jump off before my chest explodes.
That’s because at my stage I have way too much weight on the pedals vs. the seat, which is due to using legs/pedals too much to balance as I ride. The result is my legs constantly fight each other every rotation and it ramps up my heart rate big time.
I am trying very hard to get more and more weight on the seat but that is a slow process! That’s why I think it won’t be until summer before I can benefit from a larger wheel for any kind of distance riding.