Ignite Your Metabolism from Menshealth.com

It becomes more complicated when body weight is involved. For this example, we have to assume they are doing the same amount of work, ie the same power output. So lets assume they are both the same weight and are running at the same speed to equalize power output. So they will be doing the same amount of work, and therefore spending roughly the same amount of energy in doing that. Now we can compare them.
Since you understand the concept of VO2max i'll dig right in.
The elite marathon runner has a much much higher VO2max than the unfit person. How hard exercise is for a person depends on what % of their VO2max it is. So running the mile at a certain speed may be 80% of the unfit persons VO2max, but only 50% of the marathon runners, because the marathon runner is capable of performing much higher amounts of work. VO2max is a measure of how quickly you can make/burn energy not a measure of efficiency of energy. The marathon runner is capable of burning energy at a rate much much greater than the unfit person, so a certain power output will be a lower % of VO2max and therefore be easier.
I'll put it another way. Lets say that at the speed they are running, both are at 2.5 litres per minute VO2. 1 litre of O2 is 21kj of energy used. So both are using the same energy. but, the unfit person only has a VO2max of 3L/min and the marathon runner can reach 5L/min. The marathon runner thinks it is easy, because he is no where near is maximum, but the unfit person struggles.

What happens as you get fitter, is changes in the muscle mitochondria allow you to burn fuel at a greater rate, increasing the maximum sustained power output you can produce, but it doesnt change the efficiency at which that energy is produced.

Anaerobic is only less efficient in its use of fuel because it doesn't use the fuel completely. Instead of using glucose all the way down to CO2, it only gets partially used, it gets made into lactate. It doesn't change the amount of energy you expend, it only changes the way you use glucose to create that energy.

Walking and sprinting would involve different energy expenditures. Its the total power output that is relevant, not the distance covered. Sprinting has a much higher power output, which requires more energy, so you would use more energy to cover that distance sprinting.
You're right its not quite as simply as just applying physics for running. You have to take into account mechanical efficiency as well, which varies between people, but again, not significantly. Also you have to take into account body weight between people. Its a lot harder to equalize the power output between people. But if the power output is the same, the energy expenditure will be roughly the same.

 

Ok, what's VO2max?

----
EDIT: Nvm, I found out