Out pace her, even if it kills you.

I hope you said NO. And stop wasting your life crushing on someone who is not interested in you. Spend your time getting fit, then you will get a great partner, look and feel great, and make her jealous.

Did she actually say that out loud? That’s really rude, and you should tell her so. Also, find someone nicer to be with.

Now you know how vain she is.

Either
1. say yes and outpace her
2. say no and point out how rude and degrading it is

and definitely
1. write her off.

Perhaps she said that jokingly, but actually likes you and wants you along?

How romantic. You two will be outdistanced by the rest of the group after fifteen minutes, then you will have a nice stroll alone on a beautiful day while you come back to the parking lot because there's no way you will walk the rest of the path with these blisters. It could be the beginning of something!