Heart rate while running to high?

Hi, new to the forum, and fairly new to longer distance running.
Earlier this year decided to do the Ironman in. No great long distance or ultra distance athlete. Used to be a rugby player. I am fairly big for this kind of event. I am 34, 1.83m ((6ft 12") weighing approx 105Kg (231pounds)

Now I did a fair amoun of cross training (bike, swim, run), but not very long distances but I have been going between 3 - 6 days per week, depending on the work schedule.

My question is the following: My heart rate when I jog/run is always at 80/% plus, from about 1 minute in the run and stays there, it rarely comes down to 75%. This is the case whether I do 6Km, 10Km or 21Km. I don't run at any blistering pace. It varies from 5:50min/km to 6:30min/km on the flats, and 7:00min/km - 8:00min/km on the hills
According to the training programme on some days they state that you should keep your heart rate between 65% and 75%. Now for me that would mean that I would have to have a briskish walk most of the way.

I would like to know what I am doing wrong, should I worry, should I change anything etc, etc. Please help