Target Heart Rates for the marathon

I am going to use my heart rate monitor with three heart rate targets for the start, middle and finish sections of this coming marathon. According to coach Benson on Running Times Magazine, miles 0-13 at a low heart, miles 13-20 at +3-4 bpm and miles 20-26.2 at another +3-4 bpm. The trick is to choose heart rate 1 (HR1) wisely, or to put it another way, not to go out too fast. I went out at 137 in the 2004 Boulder Backroads (BBR) marathon and felt strong at the finish. I went out at 140 in the 2005 BBR and fell apart at mile 17. Last Sunday I did a 4 mi run at 140 and was very sore for days afterwards. So I am not going to start at 140. Starting at 137 seems safe but what about 138 or 139? Am I going out too slow at 137? Three Sundays ago I ran 3.5 mi at 139 and felt fine afterwards. Was it the extra .5 mi that caused my soreness and not the one beat per minute? Right now with five weeks to go I'm settling in to train at 137, 140 and 143 bpm. I may up these by 1-2 bpm in May if this goes well.