Hi,
I have been an endurance athlete and runner for over 20 years. I am now retired from all that due to arthritis. I offer the following suggestions:
1. If you are happy with your pace of around 10 min per mile and don't want to go faster then no need to bother with interval training. Though I have to say it really does work in terms of allowing you to achieve a faster pace for longer. And you don't need more than a couple of months, three times a week to make significant progress. But it is hard!
2. You need to vary your runs in terms of terrain (flat, hills), your pace and distance. If you are going to run three times per week don't do the same thing each time. Your 6 miles in 57 mins is a good base run. Try making another run more of a speed play with some hills. Work a bit harder than your steady run for a couple of minutes and then recover with slow running or even walking for half a minute. As you get fitter the efforts become a bit longer and the rests a bit shorter.
3. The most important run is your weekly long run. You must increase from 6 miles, say by one mile per week until you reach 13 miles. YES you really should have run the distance before the event! The GOOD news is you can slow down. Your long runs are about time on your feet, not speed. So you can run these long runs at 11 min per mile, which will seem easy. This is what we want. Make sure your last long run (13 miles) is 2 weeks before the event. Reduce your mileage in the last 2 weeks, but not the frequency.
You will then start the race full of confidence and ability.
Good luck!