How to avoid energy crashes during long training sessions

Few things ruin a long session faster than an energy crash.

You start feeling good, and then suddenly:

  • your legs feel empty,

  • your pace drops,

  • your focus disappears,

  • everything feels harder than it should.

Most athletes call this “bonking.”

And in most cases, it’s avoidable.

As both an endurance athlete and coach, one of the biggest lessons I’ve learned is this:

energy crashes during long training are usually not about fitness, they’re about poor fueling, pacing, or preparation.

Why energy crashes happen

During longer sessions, your body mainly relies on:

  • glycogen (stored carbs),

  • blood glucose,

  • fat (to varying degrees).

The issue is that your glycogen stores are limited.

And if:

  • you start underfueled,

  • go too hard too early,

  • don’t take in carbs,

  • dehydrate,

…your body runs low on usable energy.

This will lead to:

  • sudden fatigue,

  • poor concentration,

  • higher perceived effort.

In longer endurance exercise, maintaining carbohydrate availability is one of the strongest predictors of sustained performance.

The biggest mistake: Waiting until you feel hungry

This is where most athletes go wrong.

They think:

  • “I feel fine now.”

  • “I’ll eat later.”

But by the time:

  • you feel hungry,

  • you feel weak,

  • you’re craving sugar,

…it’s often too late.

This is because:

  • digestion takes time,

  • blood sugar is already dropping,

  • fatigue is building.

Golden rule: fuel proactively, not reactively.

1. Start the Session Fueled

This is step one.

If you start:

  • underfed,

  • low glycogen,

  • dehydrated,

…you’re already behind.

Before long sessions:

Eat:

  • carb-rich meal/snack

  • easy to digest

Examples:

  • oats + banana

  • toast + honey

  • rice + eggs

Aim for 1–4 g carbs/kg in the hours before, depending on timing.

This is standard sports nutrition guidance for endurance exercise.

2. Fuel Early During the Session

Don’t wait.

General guideline:

60–90 min session:

Optional / light carbs

90–150 min session:

30–60 g carbs/hour

2.5+ hours session:

60–90 g carbs/hour

For some trained athletes even 90–120 g/h may work if practiced.

This is especially relevant for:

  • long rides,

  • long runs,

  • Ironman,

  • race simulations.

Mixed carbohydrate sources (glucose + fructose) can improve absorption and reduce GI distress at higher intakes.

Practical examples are:

  • gels

  • drink mix

  • bananas

  • bars

  • chews

3. Pace Properly Early On

This is huge.

Many athletes:

  • start too hard,

  • feel amazing,

  • burn matches early.

This:

  • burns glycogen faster,

  • increases lactate,

  • makes fueling harder.

Smarter is to start controlled and patient, especially in your first hour of long run or ride.

Your best long sessions usually feel almost too easy at the start.

4. Hydrate Properly

Energy crashes are often worsened by:

  • dehydration,

  • sodium loss.

Even mild dehydration:

  • raises heart rate,

  • increases effort,

  • hurts gut absorption.

Practical:

  • sip regularly

  • don’t chug late

Adjust for:

  • heat,

  • sweat rate,

  • duration.

5. Train Your Gut

This is massively overlooked.

Your gut is trainable.

If you never practice:

  • gels,

  • drinks,

  • high carb intake,

…race day can go badly.

Practice during long sessions:

  • fueling timing

  • products

  • amounts

This:

  • improves tolerance

  • improves confidence

6. Don’t Ignore Recovery Between Sessions

Crashes can also happen because you’re carrying fatigue from previous days.

If:

  • sleep is poor,

  • carbs are low,

  • recovery is weak,

…long sessions feel worse.

Sometimes the issue is not intra-workout fuel.

But it’s poor overall recovery.

Biggest mistakes athletes make

  • starting underfueled

  • eating too late

  • underdrinking

  • going too hard early

  • trying new fuel products late

  • underrecovering

My personal experience

As an endurance athlete, one of the biggest performance shifts I made was learning to stop treating fueling as an afterthought.

For a long time I thought feeling empty was just “mental toughness.”

It wasn’t.

What made the biggest difference for me was:

  • eating enough before,

  • fueling consistently during,

  • staying patient early.

That drastically improved:

  • session quality,

  • recovery,

  • confidence.

And it made long sessions far more productive.

Final takeaway

If you want to avoid energy crashes during long training, don’t wait until your body is in trouble.

Focus on:

  • starting fueled,

  • eating early,

  • pacing well,

  • hydrating,

  • practicing your strategy.

Because long sessions should build fitness instead of testing how well you can suffer.

If you want help dialing in your training, fueling, and race strategy based on your own goals, explore the coaching, courses, and resources on this site.

Because better endurance is built by managing energy smarter.