You can operate for much longer than we're told; the basic guidelines for calories needed vs. exercise time/intensity is all wrong.
I only know this, because of my Appalachian Trail thru-hike and several other long-distance hikes I've completed. When I thru-hiked the AT in 2006 I had to ration my food, since you could only carry so much and my town visits (resupplies) were anywhere for every 7 - 14-days. In the beginning I was very irritable from my low-calorie intake and my all-day walking up and down mountains. However, after about a month, I noticed a significant and incredible change in my body; my body became much more efficient and the irritability soon stopped. My intake never changed much, but my mileage and speed changed drastically. I was going much further and faster on the same calories.
I generally ate a pack of oatmeal in the morning; few handfuls of gorp during the day; and a little over a handful of rice at night. I had beef jerky and various veggies I dehydrated and threw into the rice at night for dinner. This is less than I had eaten at home before my thru-hike, yet I was never so healthy and my body was never so efficient at processing calories. However, when I did get into town I was a total eating machine. There's a name for this special type of hunger: Hiker Appetite. I could close down an all-you-can-eat buffet.
I've applied this knowledge after my hike to keep my weight down. I never eat before I go on a ride/run and I do limit my food intake, much more than I did before that hike in 2006. The thing about calorie burn, is that your body does get more efficient, if you make it, but of course there's a limit, but it's amazing how little calories you can get by on. It's a personal thing, you have to figure it out for yourself.