You don't need to stretch first. Just keep taking it slowly, as you have been doing, even if that means you can only tolerate it for a couple of minutes (you don't need to try to do 10 minutes like at PT). Your legs will not be as weak as you build up tolerance, which can take a while. I wouldn't increase the time you are on it until your legs feel better. Even then, I'd only increase it by a couple of minutes.

I think it's great that you are doing the bike, Bobby