I tend to run a lot at 6.2mph. It's a good pace and I like that one. When I'm outside, I go a little faster, but for some reason it feels easier. The other day I was feeling good and went to 6.4, and then 6.6 for about 5 minutes and it felt good, but I was working hard.
What's funny to me is that I used to start running at 6.4mph when I was in my 20s and training for 5ks. Then I'd get up to 7.0 or 7.5 for some runs. I even got up there for some of my Body for Life runs at times, cruising along and working.
But I'm older, I'm running every day, and I'm trying to maintain my body for the long hall. Not just for the next few months. So I go slower, and while I can still get up to 7.0 and run, I want to push my body, not strain it too much.
I think if you have a good body you can run every day, but you definitely have to remember that modifications are needed as you get old. I'm sure I'll be down to 5.8mph in the next decade for lots of my runs.