I live in Southern New Jersey where it is very flat. I have been riding with a group of guys every Thursday night on a paved bike trail that is 6 miles long. We do a total of 36 miles, two by two, each pair pulling for 6 miles then turning around. I am 5'6 and 138 lbs. and train about 10 hours a week on the bike. I am having trouble keeping up with these guys hammering at about 23 to 24 mph. They are quite a bit bigger than I am, averaging about 6'0" and 175 to 200 lbs. i can actually drop most of them when we do the hills, but these flat time trial type rides are kicking my butt. What can I do if anything to improve at hammering at this pace on the flats? 20-21 mph, no issue, but the 23-24 is a problem.