20mph on flat terrain with a group is not out of the relm of possibilities. I ride with what I consider a strong B group most of the time. Our terrain is usually fairly rolly. When it's my turn to pull I usually lock in at 19mph and try to keep that up for 5 to 10 minutes before I switch out and then I float in the group. When I have gone with the A group that average is usually 2 mph faster and I can only pull at that speed for a few minutes before I have to drop back and then I am usually still struggling to find a kind wheel to stick to. This time of year our club rides are usually in the 60 mile range and I am usually pretty tired after them, if I do an A group ride I am done for the day, after a B group ride I can usually get some yard work done. The only part I question in all of this is the 18 month rider part and 230 lbs, I have been riding for a lot longer than that and weigh in at 175, I ride about 125 miles/week and do two intense 17 mile training rides every week. But genetically I am not an athlete so that may put me at a severe disadvantage to the OP if he was born "lucky"