Chanda - you might know this but wasn't it average for a wagon train in the Pioneer days to USUALLY go about 25 - 30 miles per day? On a bad day w/ weather, no water or indians maybe 8-9? That's what I seem to remember. And any "out" riders - the Wagon master, the hunters, any extra horse wranglers (oops that's cattle drives) usually did many more as they ranged out looking for good trailing (or if experienced - knew the ways to go), water and next place (s) to camp, checked for good, stable water crossings etc...
I know one thing - when I went to Iowa and last year when I went to OK and I crossed those long and HUGE bridge spans over rivers - I couldn't even imagine attempting river crossings in a wagon with a pair of horses! O..M..G.. I notice a lot more when I drive a vehicle now and compare it to driving the horses!
I know that distance is also relative. As your horse/donkeys get more fit, they can cover more distance at better speeds, however, there's nothing wrong with walking. No different really than you - if you set up an exercise or walking program for yourself. A couple of times, we've covered a lot of distance - some of that at a good clip and others we haven't done much at all. Terrain and how your equipment is working also plays a part as I found out with a green mare last weekend...
The one mare in the pair was jigging and acting the fool, while the 2nd mare held steady and helped steady her. The "jiggy" one made not only herself sore but also made the other mare, who bore the brunt of the "abuse", sore... Part of me was wishing I had some one at home that could hook up the horse trailer and come get us - but I'd kept going on the way out (straight) and there were no shortcuts to get back. That last mile was "killer" for all 3 of us. We did a total of 5 - we've done many more than that but she/they had had too much time off. I could barely move my arms and my chest ached all day the next day, too. I made sure I used plenty of liniment on both mares - shoulders, necks and backs - and they were cold hosed a couple of times dureing the week even though they didn't really do any work. Now I need to work with "jiggy" (might give her the new name) alone and see if I can get her to stop with all the fussing. Lord knows I did that with the mare she's currently partnered with for months in 2010 and early 2011, LOL.