THE HISTORIAN made a great point. They waited for you and worked with you. Sounds like a great group. Most clubs that post rides give contact numbers for ride leaders. Next time feel free to call the leader and ask about hills. Also remember that there is no problem or disgrace leaving the ride and making the left to go back rather than making the right. Just let the leader know. I'd bet that one of the experienced riders might likely make that left with you to make sure you get back in good shape. This happened to me on my first 30 miler when I tried to keep up with the roadies on my hybrid. I was embarrassed to be accompanied home via a shortcut and when I got off the bike I felt like my legs were made of rubber. Then I realized what a great group I'd joined. I'm still not in the best of shape (it's tough to start cycling in your mid 50's) but I now love leading slow family friendly rides that cater to new and inexperienced riders.
I would stick with the group and talk to them about other rides they do. Good luck.