We lived in Boulder/Longmont area for a long time and to be honest I think you would be better being north of Boulder versus South. Longmont is a nice area and has tons of riding from the house both Road and Gravel that I think would be much better than south of Boulder.
We ended up moving south of Denver 4 years ago on the southwest part of Parker and was very concerned about where to ride when we moved not knowing the area. To be honest, I feel like where we live now is almost better than the riding when we lived in both Boulder and Longmont. We are a couple miles from Cherry Creek bike path which goes for 100 of miles with a bunch of options and we are a few miles from some mountain bike trails (Hidden Mesa) which are not extensive but fun trails. If I ride south about 10 miles to Castlewood Canyon there are some amazing gravel roads with virtually no traffic that go forever. Some of the best gravel roads I have road and very hilly. The only riding I miss with Boulder are the long sustained climbs. It is very hilly where we live now but the longest climb takes me about 10-12 minutes at full effort. Otherwise, the riding is amazing.
Edit: Forgot to mention but Wednesday Morning Velo is a popular weekly group ride with various levels for both gravel and road. Full Cycle also has a lot of popular group rides (both gravel and road) they do weekly.