This is a hard one for most people, including myself. You don't want to disappoint somebody, but yet you already have too much on your plate. We have to remember that we cannot please everyone. Doing so is failure to define what is important to you. Saying no isn't necessarily a selfish thing; you need to draw the line between what is considered helping people and being used.
Here are a few ways you can say "no" without hurting anyone's feelings.
Saying no is tough to do and I'll be the first to admit it. However, the more you do it the easier it gets. Do it in a graceful and compassionate way and you will not only have a positive experience, but you will create more time to spend exercising.