Haha. Thanks Bob.....
It is an interesting question for physics nerds (of which I am one...), but I agree that if you want a real answer it is actually a question for biomechanists and physiologists. What the OP asked boils down to running economy, fitness level, efficiency, etc.
Ever had a day when even though everything seems like it should be fine, you just don't have the legs? We call it an "off" day and they are quite common across all athletic pursuits, regardless of level. It is also a common maxim that it is MUCH harder to run a 6 hour marathon than a sub-three, but that 6-hour "runner" is incapable of actually performing at the level required to run a sub-three. He doesn't have the mechanics, the VO2 max, the gait, the stride, the percentage of fast vs slow twitch muscle fibers, etc. Yet the amount of damage to his body, the energy expended, etc is FAR greater for the poor slow poke the back (again...count me in!) than the elite guy at the front.
If we were to take only mechanics into consideration, none of this would be true. It works great for all those calculations of energy and work and mass and time etc, but unfortunately our bodies have a heck of a lot more going on than simple Newtonian physics. Well, actually it really is all just Newtonian physics...but a whole mess of it!!!!
Here is an article from the NY Times that is a good summary: http://www.nytimes.com/2007/10/11/fashion/11Best.html?pagewanted=all&_r=0
It does a good job of explaining all the variables to running efficiency: fitness, running economy etc.
This is a huge area of research right now for those of us who work with athletes.