I don't think you need to worry about Sarah at all - she's coordinating group rides, so if you drop out - she's still got people to bike with.

Not that you need an alibi, but it should be perfectly reasonable to tell her, you've got a job, children, and a husband - when you have time to ride it's precious and you need to make the most of it right now.

I'm not sure why it's actually a problem - if I bike with someone that's faster than me, I feel absolutely terrible for slowing them down and do my best to keep up. I will stop and take a break if I know imminent bonking is about to happen or I feel like I'm going to over heat - but if I"m taking a break, I tell them to ride up ahead without me. It pushes me a bit as a cyclist - and then I make sure to give them about 20 chances to drop out if they offer to ride with me again.

I have a hard time riding with someone a slower than me - it's okay if you're just tooling around town and talking... but I can't get a workout out of it and it seems to hurt my "training/performance" over all if I do it too often.