why can I bike for miles, but running I have trouble going 100 meters...or less?

[run4him]

I had been running for years (54 year old male, fairly healthy) including some ultra marathons and triathlons.  Over the past 3 years I have gotten progressively slower and with less mileage to where my runs are only jog-walks where my jog is anywhere from about 100 meters to 100 feet before I have to walk - then I continue on as a jog-walk.  I had a recent incident where I tried to go a few extra meters (pushed it) and passed out on the sidewalk (not the first time and definitely not fun).  But I can bicycle with the same amount of water and nutrition for 20-30 miles without issue.  It's very strange and doctors can't seem to figure out why.  Note:  I have a bicuspid aortic valve that has some narrowing, but I've done multiple invasive tests, and they see it functioning fine with no heart blockages whatsoever, and not anywhere near ready for replacement.  Another history piece if it helps figure out what is going on is I did crack C2 in a ski accident 36 years ago.  There may be other things too, but hopefully this much can give someone enough information to point me in the right direction.  Thanks in advance.

[Paxiled]

I'm assuming you're not stopping because of pain but because of being out of breath.  If you hadn't seen so many docs already, I'd say to look into whether your heart is functioning properly or needs a pacemaker.  Bicycling is a whole lot easier than running -- you have a machine helping you even if it seems hard.  When I had to stop running due to a knee injury, I had to bike really hard for 3 hours just to get something like the burn of an hour long run.  It's also hard to keep up a steady rate when you're biking -- lots of stops and slowing down and such due to other people and stop signs.  But it shouldn't be that much different.  Blacking out is a sign of a slow heart that isn't pumping enough blood through the body -- my brother discovered he needed a pacemaker when he passed out going to the bathroom, which is often how it goes.  I had to stop running and biking due to pain, but your case is odd, to say the least.  Do you ever find yourself feeling faint doing anything else?