Terrain is the big factor here, along with the load bearing issue. In the mountains with full kit, pace goes way down. 2mph would be a ballpark figure there. I would take that down to 1mph or less if you're not on switchbacks and trails and trying to go uphill.

Rolling grassland is very different than rolling terrain with bushes and trees that are about 6ft in height. You lose a lot of straight line distance by veering around objects, so that hurts the pace.

Standard pace on roads is 4mph. Not too bad. Not easy, not hard. For me, being shorter, after the first 6 miles I have to start the run a bit, walk a bit in order to stay at 4mph. My stride isn't that long and I don't have that ability to move in short but quick strides. Ken White is on the money about training. If I go out walking / hiking / rucking and I haven't done it in a while I'll move about 3.5mph and feel ok. After I do it a time or two, my pace goes to 3.8mph. I then have to remind myself to pick it up a bit to get to 4 or higher. With conditioning and practice it gets easier. There is definitely some skill factor to go along with the conditioning factor. Units that train for it will do much better, as Ken White stated.

Load plan is a big deal too. Weight on my hips makes mountain movement a bit easier than having it top-loaded on vest and pack. Keeps my COG where it should be. Feels much more efficient and biomechanically I believe adding 1lb on your feet is equivalent to adding 6lbs at the waist. More reason to pick your boots wisely. So, hip loads are more efficient. However, in complex terrain they can interfere a bit with moving your legs and feet over obstacles.

Bottom line, these days, if you want to get somewhere on foot and do it tactically you'd better plan to do it slowly. Tactical movement plus heavy loads equals slow movement. You can go quickly but it won't be tactical. It'll be loud, the dispersion will be awful, overwatch will disappear, and your guys are likely to be combat-ineffective at RP. Look at pictures of squads/plts in AFG. Many times you see them on a switchback or narrow trail, just walking. Probably making decent time but you can tell the security isn't there, they're spaced about 3m apart and the overwatch is not present. Again, they are making decent time from A to B, but tactically moving takes time and in AFG most of that should be tactical with overwatch, not a traveling movement.

Finally, if you track the studs that do the Bataan Memorial Death March in the heavy category (min 35lbs) you'll see some people out there do the full 26.2 miles in 6hrs. I've done it a few times now and my range is 7:45 to 8:15. When I finish I'm tired and certainly am glad to finish. However, if I wanted to stretch it out to a 12 hour active period (dawn to dusk, roughly) I could foresee getting in 30 miles per day for well conditioned individuals, with 8 hours or so of rest, and the other 4 in prep time, camp set up, security, gather food, water, etc.