Riding I was gonna say 40 calories a mile...so 2 hour bike ride at 15 mph = 1200 calories.
Now two hours of running at say 12 min mile= oh dear 10 miles running. I bet I would burn just a friggin ton! A half hour run is pure torture....so I figure you have to ride twice as far biking as running. No science to it = just my thoughts....but all in all I think once you have been biking for a while as a fat woman (See not every one can do that) then try running it is a very very humbling experience.
OK You convinced me...I think Iwill go for a run now...(Only cause i do not have a bike here at my virtual lcoation)