determining max HR and aerobic zone training..HELP

HELP,

I am training for my first 1/2 IM.  I have done a few sprints over the years, am 41, 5'-11", 200# and loosing, 185-190 is my "fighting weight".  I am in fairly decent shape, have been running 2-4 mi 2-3 times/week for a couple of months, biking min of 25 mi/week weather permitting, some weights, etc.  I started a 19-week 1/2 IM program this week. 

My issue is i read conflicting info on how best to determine my max HR so i can establish what my best aerobic zone is for training.  I think my "aerobic zone" s/b 140-150 which i have no problem staying under swimming, biking, or weight traning.  However running i cant get under this.  I am down to an 11.5 minute pace and still go over 150 on runs over a mile.  If i run 3-4 my avg hr will be mid to high 140's but numerous times i have to walk to bring it back under 150........and even there i am guessing i am at/over the high end of where i s/b training.  I feel fine the next day.  Should i start walking, keep doing what i am doing or what?  I have a modest goal of completeing the run under 10min miles but at this rate it will never happen!!!

suggestions?