Nina,
Thanks for checking in and using our plans. Normally, I’d ask some questions like, what is your LTHR, how did you determine it, when did you determine it… but if you are spiking HR at an 18-minute mile I don’t think we will fix it.
So, don’t use HR. Use Pace. There is no reason to think you don’t have a good aerobic system, especially given your experience. Some people just have an aerobic system that has elevated HR no matter what. There is nothing wrong with that, except when you try and use it to measure intensity.
Remember: HR is not an intensity. It is an indicator of how the body is responding to a stimulus, and a bad indicator. Pace and Power are outcomes and outputs, much better measures of intensity.
It’s possible that after another year of 80/20 training you actually will make a significant shift in your aerobic systems and your HR will stop spiking, and you can go back to HR. My experience is that very few athletes who try Pace ever go back to HR as a primary.
David