Yup I mix it up, but it's usually around 6 miles. My longer routes are 10-12, and I have many routes depending on what I want to work on/feel like (hills, woods, track). On an average run, I'll do 5 minutes of jogging separated by 30 seconds of sprinting, and go all-out the last mile or so.
One track exercise I really like is to warm up with a mile run, then jog 100 yards, sprint 100 yards; jog 200 yards, sprint 200 yards; and finally jog 400 yards, "sprint" 400 yards.