Hey 7, supporting you in your brave decision to be sober. This is a big crazy hard thing we're doing but like you I'm finding the SR community to be incredibly supportive.
I think bowing out of the weight loss challenge is a great idea to help you focus on your long-term sobriety. Good on your for telling your friend you are withdrawing. My experience is that people are pretty self-centered on themselves. Your nosy friend and anyone else who is interested in why you are withdrawing will probably move onto other things quickly.
Glad you are posting! Please keep posting!