+ 1 for MAF / phil mafeton based heart rate caps for running for IM training.
I started running regularly 8 years ago. For the first 4-5, my hr would hit 155 within the first minute, and I would run 5-10k at hr 155 - 165. I thought I was fit as I was pooped at the end.
3-4 years ago, IM training started, and I started building a long run from an hour up to 2.5 hrs - at a hr of 135, (per friel / gordo / zone 2 based on a functional threashold hr (aka hard 30 min run ) of 160. As a result, my HR for a normal 5-10 k training run plumeted to 140-145. My running partner's garmin HR warning used to go off on an uphill section, and my hr was 145...!!!! wtf? I had been running with him for the last 5 years..... my hr fell, his had not......
so for the last 2 years I have had a MAF HR cap of 143 for all runs - When my daily 45 min runs started off, I was bumping this constantly. 4 months later, (5 runs a week) I really had to concentrate / push to hit 143, and a interval session of MAF -5 / +5 was hard / very hard. I ended up with 7 min miling at MAF.
(Every 2 months I do a MAF test - run 5 miles at MAF hr - 143 for me) and see what your mile times are - they will degrade when you start despite the constant HR. When you are really fit, your speed will have increased at the MAF HR, and you will no longer degrade your mins/mile.
Oh the last thing......
its an IM you are training for...... You have just done 5.5 - 7.5 hrs on the bike, and your legs are knackered.
How hard / high a HR can you push in an IM marathon. You will not be doing a tempo workout!
IM regensburg - I ran a 3.45ish. the first 2 hours I was at a HR of 150 and all was good, then it got really hard, and my HR (and speed) fell, down to 140. I just could not go any faster as the legs were so screwed, they could not make the hr rise any more.....
IM Lanza - I ran 3:49, just ran easy at 140-143 the whole way. The last 5k was tough but speed did not really suffer....
If you are less fit, then the bike will have taken more out of you, and your legs (and heart) will be in an even worse state. Its then all about efficiency