You forgot the run action!
A red dragon of old age or older, flying in a straight line, with a light load or less, moves at 91 mph. The youngest wyrmling flies at 68 mph.
200' x 4 run per round = 800' per 6 sec = 480,000' per hour = 91 mph
150' x 4 run per round = 600' per 6 sec = 360,000'...