14/05/2026
Most people train without knowing what their race actually demands from their body.
They just run, lift, and hope for the best.
But your body runs on three energy systems.
And depending on the distance you're training for, each one plays a very different role.
Knowing this changes how you should structure your training.
Here's a quick breakdown:
The first system, ATP-PCr system, fires in the first 10 seconds of hard effort.
Think explosive sprints, a fast start in a race, or a short hill sprint.
It burns through fuel fast and then hands over to the next system.
You train this with short, all-out efforts - strides, 8 to 10 second hill sprints, with full recovery between each one.
The second, the glycolytic system, kicks in from around 10 seconds to about 2 minutes of hard effort.
It runs without oxygen and produces lactate as a byproduct.
This system matters a lot for shorter, faster races like 800m to 5K.
You train it with sustained high intensity intervals - think 400m to 800m repeats at hard effort with solid rest.
The third, the aerobic system, runs everything else.
About 75 seconds of effort onwards, your body shifts predominantly aerobic.
- A 5K is roughly 95% aerobic.
- A half marathon is close to 98%.
- A marathon is almost entirely aerobic.
Even an 800m race is around 60 to 70% aerobic - more than most people think.
So what does this mean for your training?
If you're racing 5K, you need:
- A strong aerobic base
- Dedicated VO2 max work
- High intensity intervals to build the top end.
If you're racing half marathon:
- Lactate threshold becomes the priority
- Tempo runs and threshold intervals need to be a regular part of your week
- Zone 2 volume underpins everything
If you're racing marathon or beyond:
- Aerobic base is almost everything
- Majority (80%) of your training should be easy
- Threshold work layered in (20%)
Look at your goal race.
Work out what energy system it relies on most.
Then make sure your training actually reflects that.
A lot of people train hard without training smart.
Knowing your energy systems is how you close that gap.