I made a small mistake on my daily run, and it is costing me two years.

Today is day 2155 of my running streak. I have run 16,548.89km so far, which leaves 23,526.11km to complete the 40,075km distance of a lap of the world. I run in barefoot-style footwear, one run every day, and I film each one. The whole challenge raises money for children's causes, with a target of £1M.

This post is about the arithmetic I avoided for years.

What I got wrong

When I started, I set a finish date of my 50th birthday. I was 38. From my start date, that gave me 4,292 days. I worked out the daily average I needed and landed on roughly nine kilometres a day.

In the early days, hitting that average was easy. The novelty carried me. I also set myself a rule: never less than 5km a day. I allowed myself to drop to that distance in the build-up to big events, such as marathons and ultras. I have run 13 of those during the streak so far. My thinking was simple. If I run short now, I will make it up later.

I never made it up. Over time my daily average settled at 7.68km. That is a decent distance, a little under five miles. It felt solid, so I stopped checking it against the plan.

What I noticed

Recently I looked at my statistics properly. At day 2155, I am past halfway in days. Halfway through 4,292 days is 2,146, so on the calendar I am just over 50 percent of the way. In distance, I am at about 41 percent. If I were on plan, I would be over 20,000km. I am at 16,548.89km.

Running 1km to 1.5km below the average I needed, every day for six years, adds up. The shortfall comes to roughly 3,000km. The daily gap is about 1.3km. That is around five minutes a day. On any single day I could not have told you it mattered.

The options

To catch up and still finish on my 50th birthday, I would need to average more than 10km every day for the rest of the streak. I looked at that honestly.

Running 7.68km a day has already been hard on my body. Across this streak I have run through a broken foot, plantar fasciitis, three different calf tears, hamstring tears and lower back strains. I am 44 now. Raising my daily average by a third and holding it for years is a plan that ends with an injury I cannot run through. If the streak ends, the mission ends, and so does the fundraising.

So I did not choose to catch up. I changed the finish line.

The new plan

I will finish on day 5,000, which falls around February 2034. That is nearly two years later than planned. It also means the daily average I need rises only from 7.68km to a little over 8km. My new rule is 8km minimum a day, except in the build-up to a marathon or ultra, or in the recovery after one.

The 50th birthday mattered to me personally, but it was never a milestone in the challenge itself. Day 5,000 is a round number and a clearer one to track. I am comfortable with the trade.

The lesson

Small shortfalls compound, the same way small gains do. I invest in property, and I talk about compounding interest often. I had applied the idea to money and ignored it on the road. A 1.3km gap looks like nothing on one day. Repeated for six years, it moves a finish date by two years.

Three things I am taking from this:

Check the average against the plan on a schedule, not when I happen to remember.

Do not rely on making it up later. Later is when you are tired, older and carrying whatever injury has arrived by then.

Pick the adjustment that keeps you running. A smaller correction made early and held for years beats a large one that breaks you.

Consistency still matters more to me than intensity. The streak is the point. Eight kilometres a day for the next eight years or so is something my body can keep doing.

Why the mission is unchanged

The distance is still 40,075km. The goal is still £1M for children's causes. The only change is the date. The more people who watch these runs, the more support the fundraising receives, and that is why I keep showing up every day, including the short ones.

I do not plan to stop at one lap of the world. I intend to keep running until my body tells me to stop. If I had to write my own gravestone, it would say one thing: he ran the day he died.

If you run endurance races, check your own average against your own plan this week. If you want to follow the next 2,845 days, watch the episodes, subscribe, and share one with someone who might want to support the mission.

Watch the full episode: https://youtu.be/dPLUBRBzsKs