So i just ran to HR. Surely if i'd gone by pace i'd have crashed. I've seen a few people say don't go by HR in a race and i have to admit i'm struggling to understand the reasoning in that. 
Sorry, lurking as a former Hadd-er and had to chip in here! Never ever run a race by HR Mace - the nerves/adrenaline will cause your HR to be higher than normal therefore you'll run slower than you need to to keep it in check. So no, if you'd gone by pace, as long as that pace was realistic, you wouldn't have crashed. HR is handy for determining what that pace should be though. For example, I know from experience that I can a run a HM at a pace that in training is consistent with a HR of 178-179bpm (I've got a high max of 196). So, before my recent HM I did a 6-mile HM pace run with my HR monitor and came out at 6:39mm average for 178bpm. That gave me the confidence to go for that pace at GER on Sunday and I managed a big new PB, Garmin pace 6:38 av, so pretty much spot on with what my test run told me I could do. There was no need to wear the monitor on the day and it would have just been a distraction.
Another interesting point from my Hadd experiment was that although it was a success and my HR/pace improved dramatically over time, when I ditched the Hadd schedule, cut my mileage and starting doing lots of intense tempos/speedwork and races, I still wore my HR for easy runs, and that HR/pace relationship continued to improve dramatically. So, I guess what I'm saying, is don't believe what you've read about anaerobic work destroying your aerobic base or whatever, as, in my case, it certainly didn't.
Edited: 18/10/2012 at 13:01