I've gone a few times to use a bike before class doing the hill repeats that tri coach advised (I only went to a seminar, I'm not ready to pay for coaching) and today tried hanging on for the class that started.
I was already fried and he immediately started high-cadence out-of the saddle stuff.
I walked out on them 10 minutes into the class. I hate, hate, hate it when it isn't anything like on the road.