To make this map to the real world, terrain must be taken into account. Obviously donkeys can haul more weight over more distance on flat ground than climbing a

To make this map to the real world, terrain must be taken into account. Obviously donkeys can haul more weight over more distance on flat ground than climbing a mountain pass. What's the approach to doing that? Make the delta and v terms functions of the terrain? Re-derive the equation based on energy rather than distance?