HR levels in marathon races?

[Fatih Quadworks]

Hi guys,
You know trainer Dr. Jack Daniels says the effort for marathon is at 82-88%maxHR.
And I had my PB (2:46) by following this idea.
In my last marathon, started out at 85%, then 88 with one or two peaks around 90-91%, but mostly at 88%maxHR until I had a side stitch at km 14, where I had to slow down till the finish line and finished in 2:55, while aiming for 2:42 at the beginning.

But I also had done something new, agressive fueling; liquid energy gels (High5) every 20-25min, instead of GU or Roctane or other sticky gels every 40-45min with water. Also had 200mg caffeine (not from coffee) in my pre-race breakfast, 2.5h prior to my race. (I had tried the same amount of caffeine in my 2:46 marathon too)

So I think, only my starting out with high intensity caused that side stitch and bad time. If I would start at 80%, then increase it gradually, like 81-82-83, up to 85, and leave the 88-90 levels to the last km of my race, I might have achieved my goal, right?

Most of us know, following only pace may fool us, so HR focused pacing is much more safe.

How is your effort plan for marathons?
Any ideas?
Thanks.

[Sasha Pachev]

I think following HR can also fool you because HR measurement could be off. I think your most sure guide is run by feel. To get good at it take experience - lots of tempo runs, lots of races. Eventually you will learn your body well enough.