I run some mornings maybe 4-5 miles with half minute water breaks every mile, and it definetly wears me out more than a 30 mile bike ride, fixed. Running 400m, 800m, and relays in track is not as draining as say, 3 minutes of heavy sprinting on a bike.

agree with cc, the curve goes up quite a bit - distance running drains you so much more than biking. not that ive ever done a century yet.

and there is no starting ratio (1:3, 1:5). Someone mentioned before some people can do centuries but cant run a mile in ten mins. that is completely true, sometimes the other way around.

person-to-person, varies between distances.

done.