Master Your Marathon Fueling: A Complete 4-Week Guide

You’re four weeks out of your big race. Your training is almost complete, and the race-day details, your shoes, your pace strategy, your travel plans, are dominating your mind. But ask any marathon veteran: the most critical piece of “gear” you need to master is your fueling plan.

You can have the best training block and the lightest shoes, but if you haven’t rehearsed your in-run nutrition, you risk hitting the infamous wall. We recently sat down with Coach Alex Mororrow of Resolute Running (Birmingham, AL) on the Mealvana Endurance Podcast to discuss about how to eliminate this risk and turn your final long runs into successful fueling rehearsals.

The math is simple: Why Fueling during the run is non-negotiable.

Sports physiology research finds that the body’s primary high-intensity fuel source, muscle glycogen, stores roughly 1,600 to 2,000 kcal [1]. The average marathon runner burns approximately 100 calories per mile.

With 2,000 calories divided by 100 calories per mile (2000/100=20), we know that our internal fuel tank is typically depleted by mile 20. This is why most runners described as, “hit the wall”, between mile 18 and mile 20. All of your gels, chews, liquids, whatever you brought with you on the run, is not to replace all the calories you burn, but to maintain blood sugar and spare your internal glycogen stores, and pushing that “bonk” point far past the finish line.

Plan Your Sodium and Electrolyte Intake

If you are three to four weeks till your race day, it’s crucial to make fluid and electrolyte management plan ahead. Your body is still under high training stress. You need to find a plan that work specifically for your body. And your final peak runs are the last chance to fine-tune this essential element. Research shows that adequate sodium replacement is crucial for maintaining plasma volume and preventing common issues like muscle cramping and hyponatremia (low blood sodium). Current guidelines often recommend fluid containing 0.5 to 0.7 grams of sodium per liter of water to optimize fluid absorption and counter sweat losses [2].

• Determine Your Need: If you see white, crusty marks on your face, cap, or clothing after a run, you could be a “salty sweater.” While sports drinks often cover basic needs, heavy sweaters need to supplement to prevent cramping and fatigue.
• Test Electrolyte Supplements: If you suspect you need more than a sports drink, now is the time to buy and test sodium/salt tablets or high-sodium chews. Do not wait until race day to find out if they cause GI upset.
• Practice with Course Nutrition (The Warning): If you plan to rely on a course-provided sports drink or gel, you must purchase and practice with that exact brand and flavor now. Coach Alex warned against relying entirely on course support after an incident where a station ran out of fuel, causing a critical race deficit: “I tell people practice with what you love, find what you love, and then bring it with you.”

Calculate Your Personalized Fuel Plan

You can use the following formulas to estimate your exact fueling and hydration needs based on your body weight, sweat rate, and race duration.

1. Carbohydrates (Energy Needs)

For endurance events longer than 90 minutes:

\[\text{Carbs per hour (g/hr)} = 0.7 \times \text{Body Weight (kg)}\]

Example: A 65 kg runner → 0.7 × 65 = 45 g/hr of carbs

• That’s about 1–1.5 gels or chews every 30–40 minutes.
• If your gut is well-trained, you can increase to 60–90 g/hr for marathon pace or faster efforts.

2. Sodium (Electrolyte Needs)

Sweat sodium concentration varies, but most runners lose 500–1000 mg sodium per liter of sweat. Estimate your needs using sweat rate and sweat sodium test (or a mid-range assumption if unknown):

\[\text{Sodium per hour (mg/hr)} = \text{Sweat rate (L/hr)} \times \text{Sweat sodium concentration (mg/L)}\]

Example: If you sweat 1 L/hr and are a moderate sweater (800 mg/L):

→ 1 × 800 = 800 mg sodium/hr

Use a mix of sports drinks and salt tablets to meet that target.

3. Fluid (Hydration Needs)

Measure your sweat rate by weighing yourself before and after a one-hour run in similar conditions:

\[\text{Sweat rate (L/hr)} = (\text{Weight loss (kg)} + \text{Fluid intake (L)}) / \text{Time (hr)}\]

Plan to replace 60–80 % of that rate per hour during the race to avoid both dehydration and overhydration.

Example: If you lose 1.2 L/hr, aim for 0.7–0.9 L/hr (roughly 700–900 mL/hr).

Quick Reference Summary

Metric	Formula	Typical Range	Example (65 kg runner)
Carbs	0.7 × body weight (kg) = g/hr	40–90 g/hr	45 g/hr
Sodium	sweat rate (L/hr) × sweat sodium (mg/L)	500–1000 mg/L	800 mg/hr
Fluids	(weight loss + intake)/time	0.6–1.0 L/hr	0.8 L/hr

Fine-tune these in your final long runs—your stomach is trainable, and so is your fueling plan.

Absolutely Nothing New on Race Day

We can’t stress this enough: with only a couple of weeks left, you must lock down your strategy now. The biggest race-ending mistake runners make is introducing new foods, new brands of fuel, or new gear (even new socks!) in the final few weeks. It’s simply the single biggest, most avoidable risk you can take. The research is clear: Gastrointestinal (GI) distress is the leading non-injury cause of DNF (Did Not Finish) in endurance races [3]. Your gut needs time to adapt to the specific sugar profiles in your gels or the specific composition of your sports drink. Introducing a novel food or supplement in the final week significantly increases the risk of GI upset on race day because your digestive system hasn’t adapted to it. Keep it boring!

This “nothing new” rule extends beyond fuel and into your logistics and clothing:

• Final Kit Rehearsal: This is your last chance! You need to run your final long run (even if it’s a shorter, tapered one) in the exact socks, shoes, shorts, and shirt you plan to wear on race day. This is your final chance to find any chafing points or hot spots before it’s too late.
• Pack the Essentials: If you’re traveling to the race, never put your racing shoes, shorts, or singlet in a checked bag. Coach Alex learned this the hard way after a travel incident with misplaced checked bags: “Your entire race day kit… put in your carry-on,” he advises. Don’t risk having to run 26.2 miles in a brand new, untested pair of shoes!
• Start Your Logistics Checklist: Since you can’t be adding in extra mileage, use this nervous energy to get organized. Begin your race-week logistics checklist now. Double-check that your travel and hotel are confirmed, know the Expo hours, and plan your route to the start line. It’s the small logistical wins that build mental confidence.

Next Steps for the Homestretch

By rehearsing your in-run fuel and locking down your gear now, you eliminate the “fear of the unknown” that Coach Alex describes as the biggest psychological challenge for runners.

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Ready to hear the entire game plan for the final weeks? Coach Alex dives deep into carb-loading percentages, caffeine strategy, and the power of the “fishing” game for the final 10K. Listen to the full episode here.

References

[1] Hawley, J. A., & Burke, L. M. (2010). Carbohydrate availability and exercise performance: current concepts and future challenges. European Journal of Sport Science, 10(4), 287-297.

[2] Sawka, M. N., Burke, L. M., Eichner, E. R., Maughan, R. J., Montain, S. J., & Stachenfeld, N. S. (2007). American College of Sports Medicine position stand. Exercise and fluid replacement. Medicine and Science in Sports and Exercise, 39(2), 377-390.

[3] Jeukendrup, A. E. (2010). Gastrointestinal problems in athletes: a review. European Journal of Sport Science, 10(6), 49-62.