Do you know how fast you ride? A cycling computer is a big help in letting you know what your actual pace is & will help you select which group to ride with in your local club. The clubs in our area have a classification system to help riders based on how many miles per hour they ride. Another thing to be aware of is whether the particular classification you fall into is the "can ride alone & will be dropped if too slow", or the "nobody gets dropped group". The club's website should tell you this information. I doubt if you're too slow for any group they have, there seems to be a group for just about every riding level & speed.

As for hills & building endurance, you gotta get there sometime. Just don't try to do it all at once. If you use the spin class for hard training sessions, then lighten up in between & go for rides at your own pace. Before you know it, you'll get stronger & faster!