Averaging 18 or better in the saddle is fast as long as climbing doesn't exceed 50' per mile. When climbing exceeds 100'/mile, 16 would be fast. I don't do many centuries, but an elapsed time of 12 hours or less is fast for a double, hence 6 with stops would be fairly fast for a century. You're doing well for only training for 2 months. Don't worry about it. You're doing plenty of mileage, just be careful not to overdo it. Take an easy week every 4th week. Give it 5 years. They won't be blowing by you any more. Or at least a lot fewer of them.