Long Runs

Long distance running is a new thing for me. In the past, the most I would run was about 4 miles at a time.
My goal for this summer was to do 3 runs a week - 2 short, 3 miles runs during the week and 1 longer run, 6 miles on the weekend. Then once September comes around, I will begin the training schedule for the 1/2 marathons. My biggest obstacle now that I'm trying to extend my distance past 3 miles is that I'm getting burnt out by mile 4 or 5 and finding it hard to finish. I'm worried that if I can't do the 6 miles I set out to do, how am I ever going to make it to 13.1??
So a few weeks ago, I started playing around with my run/walk intervals. I started off at 10:1 but that was still too much, then I went down to 8:1, good for short runs but my long runs still suffer. I would start off at a great pace under an 11 min mile and by the end, I'd be over a 12 min mile. I want to consistantly be at or under an 11 min mile.
So this week I went down to 4:1. I felt a little weird taking time away instead of adding more time to my run time, it kind of made me feel like a failure, but I decided I needed to find what method works for me in order to get me through the whole 13.1 miles.
I'm happy to report that 4:1 seemed to be a good ratio for meI ran 6.2 miles, aka a 10K in 1:09:59, a pace of an 11:17 mile. Which was (almost) the exact same time as my 10k last weekend  (todays was 3 seconds slower). Temp was the same as last week (but a tad more humid). BUT my splits were sooooo much better. I stayed consistantly around the 11:17 pace (except for mile 4... can't figure out why that one was slower), where as last week my splits were all over the place ranging from 10:26 up to 12:05. Felt much better this time and didn't seem to hit a wall at mile 4/5 like I usually do.