Originally Posted by
robnol
running is cardio king if u deny this ur seriously misguided.....I push out all the watts the legs can do if ur weak in cardio u will be panting like a dog...sounds like ur a dog....u .need to man up and try to run 10 miles without having a heart attack....those that cant usually talk like u do
A good indication of the intensity of a given exercise is the VO2Max achieved. It's very similar for trained cyclists and runners. If you're primarily a runner you may not be able to achieve VO2Max on the bike before your legs give out. Same thing for cyclists trying to run.
Generally, XC skiing has the highest VO2Max so more intense cardio workout than either running or cycling. Nevertheless if you want to get fast on the bike it's best to ride more. There are zero elite cyclists who use running as a primary part of their training.