The big problem for me in my last race was keeping it right down at the start. I was running at the pace I knew should have given me the heart rate but my heart rate was unusually high.
I put that down to adrenaline and quite possibly overtraining. Unfortunately I carried on at the pace I thought should have been right and blew up at 17miles big time and from 3:50 projected managed a 4:42. It also may have been due to nutrition and heat. I'm not sure.
I'll echo what Vdot51 says. Look at the marco heart rate calculator website. He breaks the marathon into 3 distinct phases of heart rate allowing for drift and two miles of very slow warm up, low HR. I ran a PB using it and felt very comfortable in the last 6miles overtaking lots of runners who were walking by then.
Sunday, I opted out of the 50k as I've not done the long run training. Managed 9th place out of 51 with a 3:37 for the 18.7 mile challenge. Map reading mainly off road round the Hertfordshire countryside. Not so much a race as a challenge. I'm happy with that. Starts me off on my marathon training for Robin Hood.