My first marathon this season is just ahead, and it’s taper time! Cutting back on distance and speedwork a bit allows my body to rest and heal up, so I’ll be at peak form on race day. The nasty cold I’ve just gotten over helped enforce that rest, at least. Not that I appreciated it.

One temptation tapering runners have is to use the extra time and energy to try something new and different. Hey, I could start those judo lessons, or try out the Eskimo Diet (mmmm….seal meat). Well, you’re not supposed to do this. Stick to the familiar. And as a serious runner (well, as someone who takes running seriously), I rigorously follow this advice.

Except when I don’t.

This week I tried something new (gasp) in my running – usually a no-no right before a race. On the other hand, this change just might win the approval of my coach. Because it involved me running slower.

First, allow me just a little history to set the context.

Last Saturday, I went out for my first group run since I’d caught that cold. Coach put me down for 14 miles at an 8:20 pace. I felt well enough but told her (and myself) I would run easy and only so long as I felt (reasonably) comfortable.

The first seven miles went fine. I felt a bit winded but chalked it up to the hills on the route. I started the route back – and heard three women coming up behind me.

Cue my stupid male instinct. Easy training run? Recovering from an illness? Screw it – no way I was gonna get chicked! So I stepped it up – for a few miles, anyway. At the final water stop I graciously let them go ahead. I’d made my point. Yeah.

It wasn’t until I checked my splits afterward that I found out what I’d really been doing. 8:20 pace? Not exactly. I’d ended up running mostly under 8:00, and during my push I’d been doing 7:30 – uphill. Well, pushing the pace is fairly routine for me, and I have to admit to taking some perverse pride in it. No harm, right? Doesn’t it show how fit I am?

And yet there are those articles that say runners often do their slow runs too fast. And for five years now Coach has been trying to get this into my thick head. But what, exactly, is “too fast”? And why is a faster pace so bad?

Well, my recent fitness tests, as well as a couple of books I’ve been reading, have finally given me something quantitative to work with. And this means adding a tool to my training that I’ve had for years, but never really figured out how to use.

The heart rate monitor.

Heart rate training is popular among elite cyclists, and to some extent among runners. Instead of running at a specific pace, you run at a specific heart rate, or in a heart rate zone. The advantage is that you can tell when you are running aerobically vs. anaerobically. The threshold is known as the Maximum Aerobic Heart Rate, or MAHR.

Why is this important? Because of the differences in how the body produces energy. When running anaerobically (above MAHR), the body uses carbohydrates for fuel, of which there is a limited supply. Eventually, fuel runs low and the runner has to slow down, or bonks.

Running aerobically (at or below MAHR), by contrast, mainly burns fat, of which the body has a much larger supply available. The longer the run, the more important this source of energy is. So ultrarunners (ahem) should really be interested in running aerobically as much as possible.

Dr. Phil Maffetone, one of the pioneers of heart rate-based training, has a method for estimating one’s MAHR. Doing the math, I came up with 131 beats per minute (BPM) as my MAHR. So for me to run aerobically, I need a pace where my heart rate stays at, or just below, 131 BPM.

What is that pace? There was just one way to find out. I strapped on the monitor, set my Garmin to display heart rate, and out I went for a six-mile run. I though I might have trouble holding a particular heart rate, but it turned out to be pretty easy.

I held 131 BPM for two miles, with splits of 9:33 and 9:43. Just for fun, I also ran one mile at 135 BPM, clocking 8:55. People often find their MAHR pace is annoyingly slow at first, but for me it was okay – very comfortable, not snail-like. Maybe over 10 miles or more it will get annoying. Just one way to find out! More to come as I continue to experiment with this.