depends. alot of runners would have trouble with the whole constantly changing intensity thing of racing. Thats the key difference between a ride and racers. If yo can jump from z4-z7 100000 times in an hour you can generally do pretty well in races. Also, fastest 5k back when i was a tri-geek ws like 18:20, and i was a piss poor cyclist for a long time. Take what you want from that.