Changing my running goals

I'm realizing that at my age, my body is reluctant to go as far as I would like it to go. I've gotten my mid and long runs up to 7 miles three times in the past year, only to find that I couldn't maintain those distances. So I've decided to change my short-time goals to be a rest run of 4 miles, a medium run of 5 miles and a long run of 7 miles. And, I'm going to mix the runs up more during the week instead of always doing a particular distance on the same day of the week.