What CRO and the London Marathon have in common — and why guessing slows you down in both.

This Sunday, over 56,000 runners will hit the streets for the 2025 London Marathon.

They’ll be running on training, not vibes.
Their shoes, pacing, hydration and fuelling? All tested.
Not a single one is turning up thinking, “Let’s see what happens.”

But in the world of websites?
That’s still the norm.

The biggest CRO mistake:

Assuming instead of testing.

We hear it all the time:

“I think this layout works.”
“This CTA feels stronger.”
“Let’s just reuse what we did last year.”

Sound familiar?

In a marathon, that mindset leads to a DNF (Did Not Finish).
In CRO, it leads to bounces, abandoned carts and lost conversions.

CRO and the London Marathon? Same energy.

Here’s where things get interesting. These two worlds — running and optimisation — have a lot in common. Most importantly?

Both are obsessed with performance.

And neither gets there by guessing.

Runners use data. So should you.

Heart rate, pace, stride length — every detail is measured.
In CRO, it’s heatmaps, scroll depth, session recordings, A/B tests.
If you’re not watching what real users do, you’re running blind.

Small tweaks, big gains.

The wrong socks can ruin a marathon.
The wrong CTA can kill your conversion rate.

Optimisation isn’t always a full redesign. Sometimes, it’s a button.
Or one less field.
Or placing the offer where people actually see it.

But you won’t know until you test it.

Testing leads to confidence.

Once you ditch gut instinct and embrace evidence, everything shifts.

• Headlines are chosen for performance, not preference.

• CTAs are positioned by scroll data, not hunches.

• Changes are made because they work — not because they feel right.

You stop assuming. You start improving.

A few quick stats to put this in perspective:

• The elite runners at the London Marathon finish in just over 2 hours.

• Your users? They’ll give you about 8 seconds.

• That’s not a lot of time for friction.

If elite runners don’t leave their race to chance, why should you leave your website to opinion?

Final takeaway:

Assumptions are the wall. Testing breaks through it.

Whether it’s the 20-mile mark or your homepage, performance comes from insight — not instinct.
So kill the guesswork, embrace the data and keep things moving.