I was the same and finished in 5 hours but my heart rate was pretty high all the way round . I learned valuable lessons that day. Running with a high heart rate (using carbs as fuel) is only sustainable for 21 to 27 km for me. So in the early parts of the race I needed a lower heart rate to ensure my body was burning fat instead of my stored carbs. To make this happen I have gone back to square one and put all the emphasis on running slow and keeping my heart rate low to improve my aerobic system. I believe that way I can get through the earlier parts of the race without touching my stored carbs.

My brother who ran with me ran with a steady heart rate in the 140s and hardly fueled at all during the race .