I think the first trick is to stop thinking in terms of 'mums'. Men don't wake up and go "how do I find my tribe of dads? Where do I go to meet dads? What kinds of dads should I hang out with? What kind of dad am I?"
I never got into the whole 'mum' thing, mum as a label, y'know? I was also young, over ten years younger than the other parents, and to me it was no big deal but they were all over it. "Oh, you have a house? Oh, is your husband wealthy? Oh, are you a bit dim about contraception? PLANNED, you say, well, aren't you a little go-getter." So, yeah, no fitting in.
I didn't make special efforts to 'meet mums'. I picked up interests and went to classes, workshops and events - photography, science, technology, programming, choir, art - and met people there. Just people. Whether or not they had children didn't matter. I wasn't out to 'meet mums' and they didn't see me as 'a mum', we were just people, chatting, making friends. Socialising. Like non-parents do.
And it's so refreshing to be talking about things other than puree and nappies. Outside nursery no one seems to want to talk about anything non-child - it's naps, it's toys, it's who's looking after who. I had a nursery mother I've never even made eye contact with come up to me the other day barking at me demanding to know what primary school we were applying to, and proceeded to list her opinions about all the local schools. I had to bite my tongue. Primary school gates, everyone might break the ice chatting about their kid, but years later and christ almighty, they're still at it. Someone talk about work! TV! Film! Music! What are you doing this weekend! ARGH.
I guess my tip would be that if you don't actually fancy this whole mumsy thing, just don't do it. Be you. You, with your friends, your life, your interests. Pursue them. You are more than a mother and it's not the sole label you should be hoping to define yourself by.