Last Fall the wife and I took a little hike to see just what kind of shape we were in, and about how many miles we could expect to cover in a day.
We took 1 quart of water between us, no food, no extra clothing and only the Leatherman on my belt for gear.
We ended up covering 30+/- miles that day, saw a couple bears and lots of deer, elk, antelope and no people. We kept a lake in sight most of the way and crossed a couple of creeks in case we really needed water. We managed to ration the quart and didn't need the lake or creek water. The route we took was in rough open country but no mountains to go over. We left our Jeep at one road and made arrangements to have someone pick us up on the next road 30 miles away.
We managed, and it was a good test to see what we're capable of, but wouldn't want to walk that far again unless we absolutely had to.
In full disclosure, I told my wife that we were going out to look for arrowheads and would be gone for only a few hours. She calls that trip a " blank blank Death March" and still hasn't fully forgiven me.