I recently got a Fitbit. I also normally run ~20 miles 5x a week, with those 20 miles split into distances from a mile to 8.5 miles.
At first I would check the Fitbit during my run. However, I found this made me anxious and generally made me push myself too hard. I would think my heart rate is too low or my mile splits seem high. After awhile I got fed up with it and forced myself to not check it at all during my runs.
Now I just listen to my body when I run, and check to see what the Fitbit says about my run afterwards. I've found I have greater peace of mind AND better results.