Anything over 5 mile and my spirits are certainly lifted for a couple of days. I love those endorphins.
In regards to wanting to be alone in the countryside. I usually do, but it is nice to bump into another walker (s) when you least expect it and exchange a few words. There was a thread recently about walking boredom. Fortunately that doesn`t happen too much to me, although towards the end of a walk, I get to yearning for home and a cup of tea sooner than I used to when I was younger.
At those times it`s lovely to have even a quick hello with like minded people. Although I much prefer coming across a stag, badger, muntjac or other wildlife, before returning to my car.