Clydesdales/Athenas (200+ lb / 91+ kg) - Kinda Silly - Why can I go longer on flats?

I have been doing 20-25-30 mile jaunts on my roadbike at a decent clip, usually burning 1000-1500 or more calories on my roadbike. At the end I feel awesome, but my legs are exhausted.

I broke my wheel (silly spokes), and haven't been in a fix-it mood lately, so I just took a climb on my mountain bike. I did a ~7 mi loop, half up hill (pretty gnarly, 6mph climb for 15 minutes). My heart rate was ~165 the entire time. Mapmyride/my garmin both say ~530 calories. I am dead exhausted (more so than a long road bike trip on fairly flat).

My question is ... Why can I burn so many more calories on long rides, but right now I dont think I could go another 10 feet after the climb? Is it simply a matter of output power (ie, pretty near to max output climb for 15 minutes, vs a 80% max effort road bike?)

Is there a benefit to the high intensity climb over a longer higher-calorie ride?

You're getting into the aerobic vs anaerobic debate.

Aerobic= lower HR and long period of activity at a lower caloric burn, but a greater % of that burn is from fat stores rather than protein.

Anaerobic= High Intensity, with a greater portion of the energy provided from protein burn.



How Cellular Metabolsism works
These both start occurring after you switch from the carb/sugar burn in the Kreb Cycle to the anaerobic cycle where you get your energy from fat and protein breakdown. The anaerobic cycle does provide 5X the energy that the Kreb cycle does, but it's a slower process to initiate. Both cycles produce a compound called ATP (Adenotriphosphate) and this powers the muscles.

When you are exercising over a longer period, at lower intensity, there is less stress on the muscle tissue, consequently less muscle breakdown. The body will pull a greater amount of it's energy from the fat, but not all of it to produce ATP and Glucose from your bodies reserves.

When you go high intensity and are operating at or near Max HR, your body thinks it's in crisis mode and it doesn't care where it gets it's energy short term, so it goes for protein because it's easier to break down than fat. It thinks it's in survival mode because you are running away from a predator, in effect, or are the predator yourself and need that burst to acquire nutrients. One of the side effects of this is during the repair process, the body concentrates on rebuilding your fast twitch muscle tissue first instead of Slow twitch.
As to which is which? On a chicken, the fast twitch muscle is the white meat and the slow twitch is the dark. Fast twitch is for explosive bursts of activity, and slow twitch is for the all day grind.

I mix it up activity wise to develop and maintain both.:D

+1 to what tom said