Totally unscientific here: I do not think you can exactly extrapolate from running to walking. And I do not think calories used is a good measure of "effort". What is left out is the aerobic "cost". Running you are already at close to your aerobic maximum, whereas walking you have a lot of room to push more aerobic demands. "Effort" is also tied to how you feel and at what point you start to hurt and psychologic burn out as well as physical burn out (hitting the wall). These are very subjective but real. Altitude and elevation gain are also left out of these calculations. As is overall fitness and getting used to a heavier pack the more you do. An added 5 pounds at the beginning of the backpack season takes much more effort to carry than the same at the end of the backpack season. I think everyone has, through experience, their own unique feel for the extra effort a heavier pack requires. Too many variables to put in a universal equation.

I wear a Garmin watch (which as been reviewed as one of the more accurate fitness watches) that estimates calories used based on "measured" heartbeat, and I find it often inaccurate. According to my calories burned, given what I actually eat I should be obese by now!