To really *know* your hiking mph, you must do it and track it with the gear you plan to carry, on similar terrain to that which you plan to cover, in the season you plan to hike in. The simplest way to do this is to walk along rural roads (very quiet and peaceful as a rule) in the same season and geographic area as your planned route. This way, you can mark your time at intersections. This allows you to easily calculate your mph later on a computer.
I am a 24-year-old white guy who lives in rural Mississippi, weighs about 140lbs and stands about 6' 3" (slender build, long limbs, athletic), and have done two hikes where I tracked my mph and water usage carefully. Both were in the middle of August and both were done along the roadside, not trails. I kept track of my time and distance by noting the time at which I started, finished, and reached various road junctions on the way. Afterwards, I calculated my total time, distance, and mph using map software (I used the "Navigator" Android app, but Google maps or something similar should also work).
The first hike took 4hr 4min, 10mi, and 56.9oz water (5.69oz per mile). I averaged 2.5mph. I stopped early because I ran out of water, so I packed enough water the next time. The 2nd hike was 21.8mi, ~9hr (sans lunch), and 3.47 quarts (5.09oz per mi) of water. I averaged 2.47mph.
Because it was the middle of summer in the deep South, I had to stop every five miles or so to avoid passing out from the heat. The terrain was usually flat, but it also rolled in gentle slopes in some spots. Based on all this, I would say that 2.5mph is the low end of what you should expect on average since hiking in the summer of a subtropical climate is not going to yield optimal results; however, 2.5mph should be effective for making rough estimates in cases where you haven't had time to track a test hike.