Yeah, the coastal group is probably the most casual, though there are plenty of more casual riders who show up for the shorter Saturday ride. Towards the fall and winter, things will probably calm down even more in terms of pace.
I think the first group ride I ever did was with the CBTC, and I easily kept up with the B group. Based on your average speeds and distances so far, the C group is probably a good place to start, but you can probably move up to the Bs easily. It is a lot easier to keep your average speeds up when you're in a nice group (particularly in that coastal wind!). If you're having trouble, you can just stay off the front of the paceline. I found that group and the weekend rides in town to be a great way to learn how to ride in a group with a lot of very stable people, and you can also quickly pick up on group ride etiquette. There are plenty of people willing to help out new riders. Just let them know that you are new and don't know where you're going and haven't done group rides.
The "beginner" ride on Saturday out of the coffee shop is 30 miles with 17-19mph pace, though the warm up heading out of town is typically slow for all groups. It is a no-drop ride, but if you will struggle a lot keeping that pace in the group, wait until your fitness improves a little more to join in. They will give you ride directions if you need them. It's at least a good way to figure out some good roads.
There's also a Wed evening ride rolling at 6:05pm out of the Bicycle Link store on MLK downtown (near Jones St. and I-16). It is also a no-drop ride, though the pace may be slightly faster than the beginner Saturday group (closer to 20mph). It's usually about 30 miles as well. It is supposed to be a recovery ride for the racers and a learning ride for new cyclists.
If you wait until the evenings to ride at Hunter, you will probably encounter other cyclists who are usually happy to chat. They will be on the road doing the loop and not the path. So long as you obey traffic laws out there and keep to the right, the MPs and drivers will be very friendly.
I believe there's also an evening ride on Skidaway Island on Thursdays (?). Stop by Quality Bike Shop out on Montgomery Crossroads (I think) and ask the guys about it. You can also get a map for the Wed night ride (and some of the Saturday ride roads) from the Bicycle Link downtown.