Hi Amy, I can't remember if non-members can view the "In the saddle" area but that shows all the rides available. In general the average speeds are roughly C (12-13mph), B (14-16mph), A (17-18mph) whatever mph is! There should be rides of each type leaving Whittington Park at 0830 on Sundays. There are also training laps of Regent's Park on Saturdays at 8am and various mid-week rides along set routes which tend to be around B pace.
If you've ridden with a club before then you'll probably not need to do the intro to cycling although ICC still encourages everyone to come along to one as it also includes a sort of official welcome to the club. I suggest that you read the guide to group riding here if you can access it. Otherwise all rides are open to non-members, just make sure you turn up about 10mins beforehand and tell the ride leader that you're new and that you can ride in a group. You get 3 free rides before you have to join. Enjoy!