Quote Originally Posted by Naismith's rule
Allow 1 hour for every 3 miles (5 km) forward, plus ½ hour for every 1000 feet (300 metres) of ascent.
See the following website : Naismiths Rule For Height Gain or from Wikipedia

Here is an equivalent formula for fell runners from:

Running uphill: energy needs and Naismith's Rule

In this note we make two points, from which follows a suggestion for running uphill: .... (2) corresponding to Naismith's rule of 1 to 8, in whole numbers, for walking or running in mountainous country (1 foot of ascent is equivalent to 8 feet on the level), the rule for running on a treadmill is 1 to 3 and the rule for running on roads is 1 to 4 or 5.

This result suggests that, if possible, a runner should zigzag up slopes steeper than 1 in 8 on the fells and zigzag up slopes greater than 1 in 4 or 5 on roads.