There is no particular speed that a fit person should be able to maintain. Rather, the speed is generally a reflection of how you're training. When I was making a concerted effort to get faster, I could keep up at a group ride that averaged 22 mph for 25 miles. Alone, I could average 18-19 over just about any length of ride. Now, not so much because I'm not training that way.

The best way for you to get faster is to ride with people faster than you. Go with this group of guys and keep trying to keep up. Eventually you will. Intervals can also help, although I personally don't have the discipline for them.