Please try not to make generalizations based on your own physiology. What are you basing the OP physical limitation on? When I warm up, I am usually pushing into the mid to upper 20's. 17-20 IS my warm up. Plus when it's 6am and 30F, I push the pace as I can to get warmed up. noodling along at a slow pace just doesn't cut it (for me). Mostly I keep it in the small ring and just spin out at 95-100rpm till I warm up. Even so, it's easy on the flats to keep it around 20mph while "warming up".

Oh and I said it is possible--meaning possible for those who are in the right kind of shape.