When planning for a backpacking trip, you’ll want to plan your days so they are fairly even. You should consider planning on hiking time and not miles. What this means, is you have to account for cumulative elevation gain during your hikes. For example, if you hike 2.5 mph on level and downhill, and 1.5 mph when going uphill you have to take that into consideration. Uphill is slower because it takes more energy to go uphill the same distance as level. A rough rule of thumb when you’re out in the field, is that for every 1000′ of uphill you go, add 1.5 miles to your overall distance. More accurate formulas are below, but the point is if you hike 6 miles with an elevation gain of 2500′, you’re real distance traversed in time (and energy) is closer to 10 miles.
Which comes down to …
( ( 5280 * miles ) + ( elevation_gain * 8 ) ) / 5280 == Total Miles