My Favorite Fitness Marker

My favorite fitness marker isn’t V02 Max or lactate threshold pace. It isn’t calculated by mileage training load, or algorithmic score. My favorite fitness marker is the ability to maintain a sustainable pace while keeping my heart rate under 135 beats per minute.

Why 135 bpm? That’s the (admittedly) arbitrary line where we classify running as ‘easy.’ Ideally, I should be able to run at this easy effort level for several hours with a buddy while maintaining a steady conversation. In this case, easy actually means not hard. When running starts feeling challenging, it’s no longer easy. That’s the riddle at the heart of HR-based aerobic training.

When I started developing aerobic zones back in 2021, my barometer for gauging fitness became a 9-minute mile pace with a HR under 135 bpm. A faster pace with a similar level of effort represented a degree of fitness growth. Anything slower meant I had work to do. To this day, running a 9-minute easy mile remains my personal Mendoza line for gauging fitness after a layoff for injury, illness, or race recovery.

As training develops and fitness adaptations take place, that 9-minute mile pace gradually gets faster and more efficient. For a long time, 8:45 was the gold standard for what I considered, capital-F Fitness. Then it was 8:40, and during training for JFK 50 last fall, my easy running pace dropped all the way down to 8:30.

That expanded aerobic base showed in my results. I finished JFK in 8 hours and 21 minutes, smashing my 50-mile PR by more than an hour, finishing fifth in my age group, and nearly cracking Ultra Running Magazine’s top-50 performance list for male runners my age in 2025 50-milers. I’ve joked about just entering my prime now that I’m in my 50s, but it appears to be true.

During preparations for the Tamalpa Headlands 50K, I noticed my easy running pace has dropped all the way down to an 8-minute average mile with occasional dips into 7:50 land. I am quite honestly, flabbergasted at this turn of events. This latest breakthrough can be chalked up to a number of factors that we will break down in much greater detail during the weeks and months ahead. Please stay tuned. We’ve got a bunch of long-overdue changes in store for this newsletter.

For now, I think two factors stand out the most. The first is patience. This is Avery’s explanation and it’s become something of a mantra during training. Whenever I’m attacking big climbs on the treadmill, I tell myself to be patient. Whenever I’m chasing fast times and epic splits, I remind myself to be patient.

Patience is a greatly underrated running virtue. Think about all the times staying patient played out in your favor. Then think about all the times you tried to rush fitness gains. Trust your process, stay true to your grind, and everything will work out fine.

At the same time, I happen to be the father of a 12-year-old running prodigy who keeps me motivated. Over the winter, I made it a mission to keep up with him on our runs without my HR zooming into the upper zones. He likes to run a touch faster than me (understandable), and I didn’t want to be constantly trying to corral him because my HR wasn’t cooperating.

With Avery’s guidance, I began exploring the previously forbidden regions of steady state running. Representing the upper end of Zone 2/low Zone 3, many runners wind up here whether they’re trying to or not. For most runners, it’s a little too much effort to be sustainable, yet not enough to build fitness. The trick is doing it on purpose.

Over time, practicing running in a steady state zone broadened my aerobic base and improved my mental toughness. I know what it’s like to maintain a slightly more challenging level of physical effort because I do it on a regularly scheduled basis. What used to feel daunting, now feels so doable that I don’t even think about it anymore.

The proof, as always, is in the numbers. My average HR has gone down on all my runs while my pace has continued getting faster, a clear indication that all my running systems are firing at once. The killer workouts get all the attention, but it starts with keeping things sustainably easy.

• Learn your HR zones.

• Trust your watch.

• Prepare to be annoyed at how slow you’re running.

• Really annoyed.

• Make friends, not enemies, with your easy running pace.

• It’s just a barometer, not a referendum on your ability.

• Allow your easy running pace to change.

• Change is neither “good” nor “bad,” it’s just change.

• Give it 9 months to a year.

• Yes, really.