Originally Posted by HoustonB

Machka, I disagree with your anecdote that a double century is comparable in difficulty to a marathon. They are too different to compare.

During a marathon a runner will quickly (3 to 5 hours) run their body to a point of near complete depletion, during the event the focus is on staying hydrated, their is considerably less focus associated with consuming calories. The exertion level generally precludes consuming any substantial amount of calories.

A long distance cyclist on the other hand, will focus on both hydration and calories and a ride might last 12 to 24 or more hours. The exertion level relative to running a marathon is a lot lower.

I am the opposite of Zephyr11, his strength is running, my strength is cycling. I've done an IronMan, and I have a good idea of the relationship between calories burned cycling and running and the effort of each. You are seeing the ratio as being closer to 10 to 1 (running is 10 times more demanding than cycling), Zephyr11 is suggesting something closer to 3 to 1.

I think Zephyr11 is much closer to the truth.

A young in-shape club cyclist will aim to ride a 25 mile time trial in under an hour. A young in-shape club runner should be able to run sub 7 minute miles with some ease, that's 9 miles in an hour. In both cases the exertion level is likely to be slightly over 1,000 calories an hour. If you have never done any regular running that included 5k, 10k, half marathon etc. then you should try to refrain from making unqualified comparisons.

To the other poster referring to the Garmin device that computed 3,500 calories for a 3 hour ride - that thing needs some serious calibration.

To the OP: Typically what are you eating (and drinking) prior to a ride and when do you eat it? You need to give your body an hour to digest a light meal prior to a ride. Do you drink anything immediately prior to a ride? What do you consume during the ride?

My guess is the OP is riding on an empty stomach and consumes next to nothing during the ride. The 12 mph pace ought to be relaxed enough to allow you to eat and drink almost anything. I agree with Machka, that you should not feel like you are starving to death after a very light 2 hour ride. Note: I only have to look at a swimming pool and I want 2 quarter pounder's with cheese.