Alternate, standing and seated, pedaling..
I haven't ridden a bike in about 20 years, since I was 11 or 12. I recently decided to try a 45-min. Express Cycling class at my local YMCA.
I expect you also wouldn't know one saddle from another, (so won't ask)
or the adjustments that the Stationary bike you use at the Y, may have..
arrive early next time and ask the spinning class teacher to help adjust that one better..
Perhaps a BYO saddle or adding a gel saddle pad will be practical..
know what make & model the gymnasium gear is?