Don't disregard the math OR the physics, both posts are pretty solid. Or my earlier post either

The mathematical observation about average speed is easy to overlook. We tend to think that going up a hill at 5mph and back down at 35mph, that the average would be 20. That would be wrong: the average speed would be only 8.75 mph.

I think this is the formula he was looking for:

Va = (2 x Vup x Vdown)/(Vup + Vdown)

where Vup is average speed going up the hill, Vdown is average speed going down the hill, and Va is overall average speed.

It's kind of important. Even going out against the wind and back with the wind, the average speed is lower than the average of speeds. If we mess around with the formula and plug in some what-if numbers it's enough to quickly convince us that our top speed isn't nearly as significant to an average as is the bottom speed. In terms of energy expended, the physics of air drag reducing that top speed for a given effort increases the distinction even more.