I was just asked if I would go steady with my best friends x and I said yes.
but she isn't even interested in him now
the thing is she is not only really STRONG but I could really hurt her
I don't know what to do
PLEASE HELP!!

I can't make any sense out of your question. She isn't interested. She is really strong. You could hurt her. What are you asking?
I guess before trying to figure out who is strong or weak or who would beat up who, have you talked with HER yet? Don't BFs tell each other everything first?

Sounds more like you're opening a can of worms. Why not leave her alone and find someone else who isn't your best friend's ex?

I've always thought that it's a really bad idea to date a friend's ex. In my opinion you should stay loyal to your friend and find another boyfriend that no one else in your circle of friends has dated.