This is where practice comes in to play. Get yourself a handheld GPS, doesn't have to do mapping or be color or anything like that, just needs to be able to log. Go find a park with a trail, turn on your GPS, walk until tired then turn off the gos and go home. Now download/export the log file and use any of the dozens of programs or web sites that can plot and analyze your log to see how how far you went, how fast you did it, your average speed, the elevation, etc. Now compare with the route you may need to work someday to get some benchmark to compare to.