- Petzoldt's Energy Mile 
energy_mile = distance + C · elevation
where C is between 5 and 11, usually 11
- This doesn't take into account the increased pack weight required for longer hikes. (How many meals do you have to eat on the trail? Is it long enough that a tent is required?)
- This doesn't take into account whether water is available along the hike. If you have to carry all the water you need, the weight can become substantial, especially for longer hikes.
- This doesn't take into account terrain difficulty like whether scrambling is required, how much exposure there is, or whether a rope is required.