Knowing when to run and not to run

Today I ran the Essex marathon. Going into today I had a bad cough and bad hay fever plus I had run the Three Thorts marathon last week. I woke up this morning and drove to Halstead to start the race, I got it in my head even though not feeling the best I could run a p.b today - sub 3.13. The Essex marathon was where I ran my first marathon 2 years ago, I stood on the line today running my 3rd Essex marathon and 17th marathon (13 marathons, 2 x 45 milers, 1 x 50 miler and a 100 miler). On the drive up there it was raining and cloudy which I though was good as would make it cooler, when I stood on the start line the sun came out and it started to get hot unfortunately. I ran the first 9 miles at a good pace - all in the 6s and early 7 min miles then when I got to mile 10 I blew up and had to walk/jog for 3 miles. I wasn't enjoying it and got it in my head I was going to DNF. I was walking through aid stations saying where can I pull out and people said I could pull out at mile 14. When I got to mile 14 I started running, I thought in my head I am not pulling out, I am here now so I will finish. I struggled through to the end - jogging, running and walking for the last 12 miles. I came in with a time of 3.37.44. I learnt a lot today, when running consecutive week marathons you need to use them as training runs as you're never going to have the energy for a p.b and when not feeling good just don't run. I am sitting here writing this and I am happy as I finished today as could have easily pulled out which would have p8ssed me off. So breaking 3 hours 5 weeks after Umstead 100 was a silly idea. I will have to look to do that on another marathon this year or even next year. My main priorities this year now are getting into Vermont 100 in July and completing Caesars Camp 100 in October. Both these races will give me points to qualify for UTMB 100 next year and I can go through the entry process for Badwater.
Garmin Report - http://connect.garmin.com/activity/84355789