Confused but feelin fine

On friday I ran the same loop as monday and wednesday except at a target HR of 150. While the pace early on was steady I felt it dropping off towards the end of the run. I put it down to tiredness - can't believe that I get up early EVERY morning. The distance recorded was 8.53 miles (should have been 8.7 miles) in 1:04:56 =>average pave of 07:37 (actually closer to 07:28) with an average HR of 145. I am a bit confused as to how I can achieve a training pace of 07:30 with a 145 HR average when my average Marathon HR is 165 with a best pace of 07:51 (7 weeks ago). and I'm not talking about a starting HR of 145 building up to 175 over 26.2 miles - my HR for mile 1,2 & 3 was 167, 173 & 174 respectively - in miles 8,9 & 10 the HR dipped to 154, 159 & 159 only to rise to 171 for the last 2 miles. Today called for a long run (longer than last weeks 17 miles). Last night I went to the company BBQ but sensibly I took a lift with someone who was leaving early and was home by 10:30. I still managed 3 pints of Guiness and 2 plates of assorted meat - hardly the recommended diet before a long run. So this morning when I was heading out at 0730 hrs I told myself to take it easy on the effort with a view to doing the distance. I also took, for the first time since the marathon, a made up sports drink carried in my running belt. As soon as I started out I noticed that my back was getting wet from a small leak in the top of the drink container and decided to drink enough so that the leak would stop. However when I went for a drink at the end of mile 2 I noticed that I had only about 100 ml left in the 550 ml bottle with upwards of 15 planned miles to go. I decided to run on and only take a drink at Mile 10 so as to conserve my resources with a view to heading back home to take on more fluids and perhaps do a short loop. As the run went on I became more comfortable - I ran into Cork along the north quays of the River Lee crossed onto the southside of the City out to Turners Cross and followed the Marathon route for about 6 miles before heading for home. The 08:25 pace for the first few miles dropped to 08:15 by Mile 6 and 08:00 by mile 12. Spotting a marathon runner (telltale gel/drink belt) at mile 12 probably spurred me on to run faster than target pace - however I was always comfortable. Got down to 07:45 pace for mile 14 and more or less maintained a sub 08:00 pace until I got home. The drink at mile 10 was very welcome and I left a mouthful for Mile 14. I bumped into my Wife, Abina, a mile from home (she was getting the newspaper) and handed her my faulty drinks bottle with a request for a sports drink. She offered me a lift, which I reluctantly declined (had to do the distance). I finished the 18.8 miles with some stretching and the most welcome sports drink I tasted in years. All-in all I was very pleased with the run given my poor preparation and the mishap with my drink. Final time of 02:32:56 gave an average pace of 08:08 for the run with an avg HR of 131. My HR didn't go above 130 until mile 10 and averaged 143 for the last mile.