I push myself hard as I guess I believe the no pain no gain mantra , I`m only competing with the Jack Russell who is faster and has more stamina than me.Have you previously timed yourself walking over rough ground? I mean, when covering more than a few miles or other than as a military activity?
Maintaining an average of 4mph is good going, particularly if you're walking alone with nobody to compete against. That's about the same speed that a healthy person would consider a brisk walk on level footpaths.
Rather than increasing speed, increase distance. Also, if there's a health aspect, do two short laps rather than one long one. That way, if you start to falter, you've only got half the distance to go to get back to safety.