If you can hear your own breathing in the first half of a marathon, you are pushing too hard.

That is the one sign I talked about on Day 2163 of my daily running streak, and I think it is the clearest one. If you are chasing a marathon PB or a better ultra marathon performance, it is worth a few minutes of your time.

Where this comes from

I have run 13 marathons and ultra marathons so far. I broke my foot at the back end of last year, and I still aim to run at least three a year. This year I decided to give the spring season a miss. I want my foot to have the maximum time to recover before I push it to those extremes again. I will push again towards the end of the year, so this is as much a note to myself as it is advice for anyone else.

Why energy decides the race

At 5K, 10K and even half marathon distance, fuelling does not become a big issue if you have trained well. At marathon and ultra marathon distance it does. Fuelling and hydration become a large part of the race simply because of the duration.

I think of it like a fuel gauge on a car. The further you go, the lower it drops. The faster you go, the faster it drops.

Your body has two main fuel sources. In heart rate zone two, you burn mostly fat. For me, the switch happens at around 140 beats per minute. I work on the basis that I carry something like 70,000 calories in fat stores. A marathon costs me roughly 2,500 to 4,000 calories. So if I stayed under 140 beats per minute, I could keep going for a very long time.

The problem is pace. Under 140 beats per minute, I would be running somewhere between 5:40 and 6:20 per kilometre, which is roughly 9 to 10 minutes per mile. That is too slow if I am chasing a time. So I go above that line and burn carbohydrate. Carbohydrate runs out far sooner than fat, and the harder I push, the sooner it goes.

That is the real cost of going out too fast. It is not the pace itself. It is how quickly you spend a limited supply.

The breathing test

Most people watch their watch. I think the better instrument is your breathing.

For the first half of a marathon, you should not hear your breath at all. It should be controlled and rhythmic. You should be able to hold a conversation comfortably at that pace. If you cannot, you are pushing too hard, and you will not make your target.

From halfway to around 20 miles, keep the same rhythm. Coast if you can. This is the stretch where discipline matters most, because nothing dramatic is happening and it is tempting to speed up.

At 20 miles, you have 6.2 miles, or 10K, left. That is where the gasping breath is allowed to start. You push hard, you spend what you have saved, and you get to the finish line.

If you are gasping at 5K, 10K or 20K into the race, you have burned your carbohydrate too early. You will hit the wall, and from there it is a long way home. I have pushed through the wall on a couple of occasions. You can still finish, but it is horrible, and you will not hit the time you set out to run.

You will hear it at the start line of almost any marathon. A lot of people are already gasping in the first 5K. They are racing the watch and ignoring the signal their body is giving them.

What this means beyond marathons

I think this applies to more than race day. The pace on your watch is a number. Your breathing is information about your capacity, in real time. Chasing the number while ignoring the information is how you run out of energy before the finish.

That is also how I approach a daily running streak. I am on Day 2163. Consecutive days only work if no single day takes more than it should. Pushing too hard on one day costs the days that follow. Consistency comes from controlled effort, repeated, with the hard efforts saved for when they count.

The mission

This streak is part of a mission to run a lap of the world, 40,075km, in barefoot-style footwear, one day at a time. As of today I have covered 16,604.33km, which leaves 23,470.67km to go. Alongside that, I am working to raise £1M for children's causes. The more people the vlog reaches, the more funds we can raise for children. That is why I film and publish every single day.

The 40,075km only works if I protect my body for the long haul. My foot is a good reminder. Rest the spring season, recover properly, and come back for the harder races later in the year.

What to take away

Listen to your breathing. Barely hear it for the first three quarters of a marathon. Save the effort for the last 10K, and judge the first half by how easily you could talk rather than by the watch.

If you have a question about marathon pacing, fuelling or heart rate zones, leave a comment. I will answer anything relevant. If you want to support the mission, subscribe, share or comment on the video. Every extra person who sees it helps the fundraising, and I will be back tomorrow with Day 2164.

Watch the full episode: https://youtu.be/R3-J8uELFtc