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How I’ll Do It

The 5 Marathons

The first marathon will commence the morning of Wednesday 13 September and an additional marathon will be completed each day until the 5th and final marathon on Sunday 17th September.

The venue for the first 4 marathons combines both the Sydney International Regatta Centre and a loop around the Nepean River in Penrith NSW Australia. One lap of this route equates to 21.1km hence this course will be completed twice each day.

The final marathon 5 will be the official Sydney Marathon to be held on 17 September 2023. Details can be found here.

Fasting will commence the evening of Tuesday 12 September and will continue until the completion of the 5th and final marathon on Sunday 17 September. Consumption during the 5 day fast will only consist of water and mineral supplementation (sodium, magnesium and potassium). As a result, the marathons will be completed in a fasted state and without any caloric intake for the 5 days. Existing body fat will be the primary source of fuel.

How much energy is required to run 5 Marathons?

At a steady pace of 7 minutes per km (8.4km/h), the caloric requirement per 42.2km marathon is approximately 3,300 Calories (calculated here) Thus the caloric requirement for 5 marathons is roughly 16,500 Calories (which is approximately 16-20 % of available energy stores). A slower pace will reduce this caloric requirement. Approximately 25,000 Calories will be expended during the 5 day fast once the baseline daily energy expenditure is included.

Strava map of marathons 1-4
Sydney Marathon Course Map

Marathon Strategy

Whilst it is tempting to simply run as fast as possible, the chosen strategy is to ignore pace and instead concentrate on heart rate for the duration of each marathon. The goal whilst running is to ensure that average heart rate is maintained at 140 bpm. This would equate to a ‘zone 2’ level of effort which is deemed to be most efficient for ‘fat burning’.

Here is an example of heart rate during a training run:


An essential requirement to be able to complete this challenge is to become ‘fat adapted’ and conditioned to burning fat as a fuel. Alex has been adhering to a low carbohydrate high fat (LCHF) / ketogenic diet for more than 13 years and has thus been fat adapted for significant period of time. Advice from both a qualified dietician and a team of medical doctors has ensured an optimised ketogenic diet was followed.

A number of periods of 3-4 day fasts have formed part of the preparation. Fasting in conjunction with a ketogenic diet comes very naturally due to the huge level of satiety obtained from the combination of nutrient dense animal sourced foods, their high fat content and a metabolism that recognises that the body is already carrying vast amounts of stored energy. The mental effort required to maintain a prolonged fast whilst ketogenic is thus minimal.

Every training run prior to this challenge has been completed in a fasted state.
It is critical that adequate hydration is maintained for the entire 5 day challenge. Hydration requires both adequate electrolyte replenishment and, of course, water. The strategy will be to drink to thirst (and not over hydrate) and to consume between 4-6tsp of mineral salt each day.

Finally, the importance of sleep is often overlooked. The goal is for 9-11 hours’ sleep per night during the challenge to maximise recovery between marathons.


Throughout the challenge, the following measurements will be taken at least twice daily.

  • Blood Glucose (via Continuous Glucose Monitor)
  • Blood Ketones
  • Urine concentrations (via Vivoo strip) of:
    Ketones, Protein, pH, Hydration, Calcium, Magnesium, Sodium & Vitamin C
  • Weight
  • Hunger
  • Mood


Blood tests and Dexa scans will be conducted to compare full body composition and metabolic markers before and after the challenge.