You can survive it. If you do get too tired don't stop just turn your resistance down a bit. It's best to always keep moving even if it is a bit slower.
Make sure you wear something that won't chafe (bike pants or 3/4 tights) and keep in mind that the seat might leave you a bit sore after the first class or two but after that you will adapt or toughen up or something.

And yes, we do get big people in our spin class. I think some of them prefer it because it isn't as bouncy as some of the other classes.