Originally Posted by
base2
There is no comparison.
Wait, there is. It's just that time, miles, and speed fail to really capture it well and the results are so easily skewed so as to be effectively meaningless.
Running is harder. The minimum energy expenditure to run, not jog, but run is much higher than sitting on a bike. Something on the order of 100-120-ish calories per mile. To "run" a mile with reasonable enough form to actually be "running" takes ~8 minutes or so at the slowest possible pace. An hour would take you 7.5 miles and about 750-900 calories.
The same number of calories expended on a bike might very well be a personal time trial of 30+ mph average speed for an hour. Impossible. A more reasonable 18-20 mph speed is (off the cuff) ~8 calories per minute equals a depressingly low 480 calories per hour. It would take about 1 hour 45 to 2 hours of reasonably high effort cycling to equal runnings 750-900 calorie expenditure. So...36-40 high effort cycling miles in 2x the time running takes to do 7.5 miles. 40 miles in 2 hours is awfully fast.
All variable and subject to externalities, of course.
I've been told that a 4 hour marathon (slow) is about equivalent to a double century. I don't know if that is true or not. The math suggests 3500-4000 calories to do a marathon depending on fitness. Personal experience of ~7500 calories measured for a double century in 3-4x the time is close enough to be plausible from an "effort x time" perspective.
Useful? I'm not sure.
With power data you can get pretty accurate energy consumption for cycling. 29-32km/h (18-20mph) on a flat will require more than 480 calories unless you are Dan Bigham aero. Reasoning - it felt low so I found the first ride I could of mine that is on a flat. 33 minutes 29.7km/h 305kJ which is more less 1:1 to calories. That's 554 scaled to one hour. And the speed is in the lower third of your range. To get to 31km/h or even 32km/h one would need to spend disproportionately more energy to overcome draft so that would be around 600 calories per hour. To check that I actually looked for a ride where I had that speed and found it, 32.2km/h in 29:59 was 307kJ so yes, 32.2km/h for an hour would be 614 calories in this instance.
Taking your upper calorie per hour estimate for running, 900, then that means at a speed of around 32km/h a 4 hour marathon would be a 6 hour ride.
However, that is just energy. I dare say a 4 hour run is harder on the body than a 6 hour ride, if the ride is flat. I have done 5+ hour rides with climbs and I dare say they are pretty tough, but on a flat? Don't think that feels right to compare.