Walking on a smooth road (doesn't have to be metalled, just smooth) is particularly hard on not just the joints but also the muscles. The reason for this is, on a smooth surface your muslces act almost exactly the same on every step. This repetition is extremely tiring for them. Walking over slightly rougher ground is paradoxically easier. Because the ground is uneven each time you put a foot down and push off again the angle will be a little different, so your muscles are used slightly differently. You might not expect that to make much difference but believe me, on a long day, it makes an astonishingly big difference.
The normal calculation for walking times is 5kph plus 1 minute per ten metres of ascent (obviously very rough ground can slow things some). I do quite a lot of walking and I find I generally go a little faster than this, however when backpacking (carrying tent, bedding, cooker, fuel, food) I tend to go almost smack on this rate (even including breaks).
At this rate and carrying all that stuff on a long multi-day walk I generally plan on about 28 kilometres (17.4 miles) a day. I can keep that up for about 10-14 days. After that, unless I have really been working on my fitness, the biggest problem is not muscles (by then they have gone past hurting and have stared building up more stamina), but rather joints, my hips in particular, and that is caused, not by the walking itself, but rather by supporting the extra weight of the sack (around 15 kg typically) and not being accustomed to carrying that weight for so long. Again if you do this kind of waking all the time then that too would improve.
Another thing is don't necessarily figure that older people not be able to do as much. In my experience older people (up to maybe 70-75 years) if reasonably fit, can often go further though possibly a little (not much) slower. I have often been out in a mixed age group and in the morning the younger folk are itching to go faster. The older folk plod along at the same speed all day and, come the end of the day, they are still going strong, no faster, no slower, whilst the yougsters are lagging and looking distinctly ragged.
Just to give a specific example some years back I did the Coast to Coast walk. This is some 190 miles which I did in 10 days (might have been 11 with one rest day in the middle), carrying full backpack, and spending every night in the tent. The route goes over the Lake District, the Penines and the North York Moors so it does have a lot of up and down but is mostly on good paths. 3 days from the end you have to do about 25 miles across the Vale of Mowbray and, though flat, most of this is on tarmac. Both my two dogs and I had been fine up to this point but that day destroyed us (yes the dogs as well - very fit collie mongrels). Next day as we set off into the North York Moors we were definitely hobbling.