I have 2 Max Heart Rates and don't know which one to use when calculating my zones and training:

1. When I've been riding less than 1 hour - MHR=167 bpm (my age - 52)
2. When I've been riding over 3 hours - MHR=180 bpm

I understand this phenomenon is "cardiac drift" and normal.

Just got Benson's Heart Rate Training and am starting to be more focused and deliberate in how I train; however, I can't find any discussion of how cardiac drift is to be compensated for. This weekend, I was determined to find my MHR using the method prescribed in the book: 3 uphill 100% efforts for 2 minutes with a short recovery between each one, measure HR at end of 3rd try. Thought I was going to die, max HR was 167 bpm. This in contrast to two weeks ago, where I hit 180 bpm on a hill after riding 3 hours, not even at 100% effort.

Seems obvious that my max rate is 180 or higher, but if I attempt some of the drills based on that rate in the first hour, I believe they will be at higher than the intended intensity. For example, there is some speed work at 95% MHR, which at 171 is higher than I can get in the first hour. I believe there are some other drills that are intended to be below AT, but that would go above my AT, were I to use 180 as my MHR.

I'm also interested in doing LSD at 75% to build my base, but don't know where to start from, and whether I should allow my HR to drift upward over time.

Has anyone encountered this issue? Should I adjust my training zones upwards over time to compensate for cardiac drift?