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?

I think you are overestimating the effect of cardiac drift, especially in this time frame. Cardiac drift is putting out the same amount of power, but the heart rate drifting upward mainly due to dehydration (decreased stroke volume, though not necessarily from reduced blood plasma volume).

Also, you're a little fuzzy on the definition of MHR. Your MHR is just that: your absolute maximum heart rate. Your MHR is at least 180.

The biggest reason that heart rate has been supplanted by power as the training metric of choice (until I make a little more money, I'm stuck with HR, too) is that HR vs. power and RPE can vary widely from day to day, and between different times of day.

For example, I know that I feel like I'm working really, really hard at 155 on my commute at 5 am (with short sleep), but I can sustain 170's with seemingly much less effort in the afternoon. If you're unable to get your HR up to where you know it can go, you can usually figure out why. In my case, my legs often aren't fresh enough to drive my HR up that high. Additionally, RPE is known to be generally higher for the same effort when working out in the morning.

When using your heart rate, most sources will say it's better to base them on % of your HR at lactate threshold, rather than % MHR (or %HRR, which is better than directly using % MHR). Look around here and you'll find a protocol for finding that.

I don't use my HR zones for prescribing exercise to myself; I know roughly where my threshold is, and gear my rides according to it, but I'm not a slave to HR during exercise. Because of my intended use, I actually just use it as a measure of accumulated stress, similar to TSS. I use a couple of equations. One is based on % HRR, the other based on time spent in 5 zones. They are both nonlinear and weight higher HR's more heavily, but the one based on zones diverges from the other for the highest two zones. I do this because a) it generally reflects what I feel about my workouts very well and b) comparing the two tells me right away if I am hitting my planned intensity vs. volume. In the past, I have gone overboard with the intensity while trying to build volume, and I've gotten burned out. So far, this system works well (for me, as it was designed by me and for my use).

Tadawdy has it right. Look at the sticky in this forum which gives an easy way to determine your anaerobic threshold.

It's no mystery that you can't get your HR close to maximum early in a ride. Neither can I. I need a fairly extensive warming up period before I can get everything out. The more intensive the warm-up, the shorter it needs to be.

Got it -- my warm up was too easy -- I was riding with a slower partner. Thanks!