Wow, the extroverts and 'social butterflies' on here do NOT like it when people don't want to socialise with them, and prefer to stay in a very same circle of friends/family DO they? 
Weirdly, I never see introverts/people who aren't keen on socialising/joining big groups etc, slating and berating people who DO enjoy all the socialising/typical extrovert stuff.
And LMFAO at @CirclesWithinCircles calling posters 'mean girls' on here for daring to challenge her posts. So YOU can spout rude and obnoxious views, like suggesting anyone who doesn't socialise a lot will rot and wither and DIE, (accusing them of 'atrophying,') and saying they lack BASIC social skills, and are so on, but no-one is allowed to say anything back to you?!' 
Also, note to self - NEVER join a fucking runners group. Yours sounds horrendous circles. Seriously, like a bunch of controlling, manipulative bullies.
@circleswithincircles
I do have a friend who suffers from agoraphobia and possibly ASPD but certainly depression, withdrawal and associated problems, and that started out as a dislike of socialising and friends, and a refusal to make an effort. But thats an actual clinical disorder different from what the OP describes (I think) and its terribly sad to witness such a downward spiral.
Her being pushed and bullied into socialising when she doesn't want to, by pushy 'outgoing' people accusing her of being 'boring,' probably didn't help. 
To be honest @circles, I agree with the posters saying you sound pushy and rude, and your posts are offensive. Problem is, no matter how many times you read your posts back, you will NOT see how wrong you are, and how rude your posts are. People like you never do.
The thing is, some people say 'just say you don't fancy socialising.' The problem is, some people like circles won't take no for an answer, and accuse people of not wanting to socialise, of being miserable boring fuckers, who will probably never have any friends, will never be happy, and will disappear into a downwards spiral of dark deep depression. 
Oh and @paul2louise YANBU. No real suggestions though, other than tell them you love going running with them, but are happy to leave it at that. If they keep on at you. just leave the group, because they won't stop nagging you. Some 'extroverts' and 'social butterflies' will not take NO for an answer, and HATE introverts for some reason. Some of them can be real bullies.