Wow! Only your second week of riding and you're 1) already doing 20 miles; and 2) averaging almost 15 mph? You're doing awesome! Most of the groups I ride with average somewhere between 12-18 mph, which you're doing already. Unless you're wanting to ride with "the bog dogs", you'll do just fine on most recreational and intermediate rides.
And in another 2 weeks, you may be ready to race!