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.
I'm in a similar boat. The Garmin's heart rate alarm is helpful for me because I can (and do) ignore it for the most part, but if I start pushing hard it reminds me to consider if I should slow down (nearing the crest of a hill might be a reason not to). It's helpful as a tool, but it's only a tool to help me understand myself. It won't actually help me run.