I did this year was my first ironman. Blinding event, by far the best thing I've done. Ride 100 and London marathon maybe comparable but considering the time - in my case most of the day - imv2015 was a total stunner.
300 dnfs though with 35 degrees in the shade. Personally my rear derailleur went kaput 40k from the bike finish but I summoned up the spirit of the blitz and managed to finish 10 mins inside the cutoff, then ran walked out a 5 hour marathon.
I've been HR training on the turbo. This is all personal newb opinion and I haven't checked the exact boundaries but ... You can get an idea of the zones by correlating what you feel when and comparing to the HR trace. On a hard TT style ride I would have a very clear cutoff in effort capability when HR hit 140 then again at 155, which is basically the bottom and top of my zone 4. Above 155 and I'm in the red zone (=5). Below 140 and I can go all day ...
Ho hum for some reason this post truncated. Never mind ..
By the way I did the 16 week plan, only ran 3 days a week plus 1 other day swimming normally. So short, medium and long run, each week. Had run lots of half marathons before but never anything so organised. Few weeks before nailed a 22m training run on my target race pace so from then I knew I'd done it. If anything the marathon itself was just executing the plan. I almost cried on that 22m run however I was so happy to be able to run at the end and thought "I've done it!"
Did 3:56 at Brighton 2012. 99% sure had I not had a Garmin I would have come in over 4 hrs. Ran the race in my Kinvaras. Happy days lol
This is advanced, fewer days would be the target for a lower time: