Now that I'm a fitness instructor, (sport yoga), I've come to really appreciate the importance of modifications.
A lot of people are afraid to go to fitness classes because they feel they might not be up to snuff physically. It might be too hard. A good teacher will be able to tell you low impact modifications or how to lighten up on weights.
I have people in my yoga classes that can put their feet behind their ears. In the same class I have a man that can't touch his toes. So I make sure to give a few versions of the same pose so that everyone gets just the right workout for them.
So don't be afraid to speak up if something is a little too tough for you to do right now. If you have a quality instructor, they'll give you a few options to tone it down a notch or two.