Depends on what you want to do. I've trained for ironman races. Cycling is way more effective than running or swimming by themselves for cardio development for the simple reason that I can be on my bike for 5 hours and still feel fresh to train the next day, whereas a five hour run takes more out of my muscles and lowers the quality of the training the following days.
Discussion
I get that, but even if you sit on your bike for 10hrs it doesn't replace what jogging does for your body in overall health terms.
I never said you can't cycle more hours than you run, it just doesn't replace it!
Also that's awesome that you're doing Ironman races!
Which ones have you done?
Nice - underestimated the mountains plus the marathon was a death march
Frankfurt - boredom squared plus DNF because it was 40+ degrees celcius. Got 10k into the marathon and my bro who I did it with decided he'd had enough.
Hopefully more fun ones out there like Roth but kids and work get in the way these days.