Back in August I said that I needed to get better at slow running. It was a bit of a throw-away comment back then, but I am now actually focusing on slower running to improve my aerobic base so it feels like a good time to write up some thoughts about it.
Aerobic and Anaerobic Exercise
Exercise can be split into two types, aerobic and anaerobic. Aerobic exercise, something like running, is a continuous activity done at a lower, but still elevated, heart rate. It is fuelled by oxygen. Anaerobic exercise, something like weight lifting or high-intensity interval training, is a high intensity exercise which is meant to be short and fast. It is fuelled by energy stored in muscles. Aerobic exercise is good for cardiovascular health whereas anaerobic exercise is generally used to build muscle mass, help strengthen bones, and improve blood glucose control.
Heart Rate Zones
So what defines how hard your heart is working and therefore whether an activity is aerobic or anaerobic? The answer is heart rate zones and there are generally considered to be five of them:
Zone 1 is a low intensity zone (50% to 60% of your maximum heart rate) and it is good for warm-ups, cool-downs and easy workouts.
Zone 2 is a moderate intensity zone (60% to 70% of your maximum heart rate). It is good for building endurance and burning fat.
Zone 3 is a moderate to high intensity zone (70% to 80% of your maximum heart rate). It is good for increasing your aerobic fitness and strength.
Zone 4 is a high intensity zone (80% to 90% of your maximum heart rate). It is good for working towards speed and power gains. This is a glycogen (carbohydrates) and protein reserve burning zone.
Zone 5 is a very high intensity zone (90% to 100% of your maximum heart rate). It is used when you are really pushing hard and trying to do things like beat a personal best. It is not sustainable for very long.
Actual Values
Saying “60% to 70% of your maximum heart rate” is a bit meaningless and there are several ways to get actual heart rates to work with. Below are three common ones.
Rough Guide
A simple way to get a rough idea of your maximum heart rate and therefore the heart rates in each zone is to subtract your age from 220. For me this is 220 - 52 = 168.
You can then calculate the beats per minute in each zone by dividing your maximum heart rate by 100 and multiplying that by the percentages:
Zone 1: 84 to 101 BPM (50% to 60% of 168)
Zone 2: 101 to 118 BPM (60% to 70% of 168)
Zone 3: 118 to 134 BPM (70% to 80% of 168)
Zone 4: 134 to 151 BPM (80% to 90% of 168)
Zone 5: 151 to 168 BPM (90% to 100% of 168)
This is the most inaccurate method but it provides a reasonable guideline if you have no other data to work with.
A VO2 Max Test
This is a test you undertake in a controlled environment with a doctor or sports specialists. You undertake some form of cardiovascular exercise (usually running on a treadmill or riding an exercise bike) and you do this at maximum effort whilst your oxygen levels are monitored. This is the most accurate method to use.
I have booked myself one of these for the end of January 2026, a year to the day since my Zwift Ride was delivered and I started exercising regularly. I will write about the experience and results.
Smartwatches/Heart Rate Monitors
Not as good as a VO2 Max test but better than the rough guide is to use the data from a smartwatch and/or a heart rate monitor. A heart rate monitor is more accurate than a smartwatch for finding your maximum heart rate but a smartwatch will do. Smartwatches are also good for gathering information about your resting heart rate which is also needed for this calculation (although you can measure your pulse over a few days when you first wake up).
My Garmin Fenix tells me that my maximum heart rate is 187 and my resting heart rate is 48. By subtracting the resting rate from the maximum you get a heart rate reserve value which, for me, is 139.
Using a similar calculation to the one in the rough guide, you calculate the zones via the percentage of your heart rate reserve (rather than the result of subtracting your age from 220) and then you add your resting heart rate on to those figures.
Doing that I get:
Zone 1: 118 to 131 BPM (50% to 60% of 129 plus 48)
Zone 2: 131 to 145 BPM (60% to 70% of 129 plus 48)
Zone 3: 145 to 159 BPM (70% to 80% of 129 plus 48)
Zone 4: 159 to 173 BPM (80% to 90% of 129 plus 48)
Zone 5: 173 to 187 BPM (90% to 100% of 129 plus 48)
This shows that my zone 2 heart rate range is higher than the rough guide suggested.
Running In Zone 2
Having shown all of that, my Garmin Fenix does allow me to create a workout where I specify that I want to run in Zone 2 and it will do the maths for me and it alerts me when my heart rate is too high or too low. So running in Zone 2 should be easy, right?
Well for me, when I am running outside, I’m really bad at it. I always seem to end up running too fast and the hills (even fairly gentle slopes) where I live make it all a fairly stressful mental exercise as much as a physical one.
Fortunately the miserably cold, wet weather we have had recently got me to try the treadmill at my CrossFit gym and I’m getting the hang of it and am getting better about adjusting the speed to match my desired heart rate range. It’s not as engaging as running outside but, because I am running at the right sort of pace, my breathing is much more controlled.
After just a few slower runs I can feel the huge difference between the kind of running I usually do (breathing heavily, focusing on the physical act of running) and slower, zone 2, running where I can breath much more freely, feel like I could talk to someone and I can listen to and concentrate on podcasts and audiobooks. This is the kind of running I imagined before I started Couch to 5K, where every run does not feel like a race.
I am going into this phase of “exploring exercise” knowing that increasing my aerobic condition is a slow process but I am hoping that the next four wintry months will give me plenty of opportunity to get more used to running at a suitable pace on a treadmill and get a better idea of what that pace actually is so that by the time spring comes I will be able to do zone 2 runs outside much more easily.