Garmin Fenix 8 Excels in Training, But Not for 100-Mile Races

The author finds the Fenix 8 smartwatch ideal for everyday fitness tracking, but not up to the demands of an ultramarathon.

Apr. 2, 2026 at 6:18pm

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Jack Espy, an athlete training for the grueling Leadville 100 ultramarathon, has been a long-time Garmin user. He praises the Fenix 8 smartwatch for its seamless integration with fitness apps, real-time workout data, and overall reliability. However, he notes the watch's limited battery life of around 22 hours when actively using GPS features, which is not enough to last the 30-hour cutoff for the Leadville 100. As a result, Espy will be using the Garmin Enduro 3 instead, which boasts significantly longer battery life of up to 320 hours with GPS.

Why it matters

Ultramarathon runners require specialized gear that can withstand the extreme demands of races lasting over 24 hours. The author's experience highlights how even high-end fitness trackers like the Garmin Fenix 8 may fall short for the most grueling endurance events, underscoring the importance of carefully selecting the right equipment for the job.

The details

Espy has been a loyal Garmin user for nearly a decade, starting with the Fenix 5 and progressing through various models. He praises the brand's seamless integration with fitness apps, real-time workout data, and overall reliability. However, the Fenix 8's battery life of 7-10 days in smartwatch mode drops significantly to around 22 hours when actively using GPS features, which is not enough to last the 30-hour cutoff for the Leadville 100 ultramarathon. As a result, Espy will be using the Garmin Enduro 3 instead, which boasts significantly longer battery life of up to 320 hours with GPS.

  • Espy has been using Garmin watches since 2017, starting with the Fenix 5.
  • He upgraded to the Fenix 6 a couple of years later, then the Tactix 6 before settling on the Enduro in 2024.
  • Espy recently acquired the Fenix 8, which he has been using for about a year.
  • The Leadville 100 ultramarathon, which Espy is training for, is scheduled to take place in August 2026.

The players

Jack Espy

The author, an athlete and entrepreneur who is training for the Leadville 100 ultramarathon.

Duncan Callahan

Espy's running coach, a two-time winner of the Leadville 100.

Garmin

The American company that manufactures the Fenix and Enduro smartwatch lines used by Espy.

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What they’re saying

“The Fenix 8 is the ultimate ultramarathon watch. It has everything the Fenix 8 offers and then some, but the real differentiator is battery life. The last thing I want during a race like Leadville is even a hint of stress about my watch dying.”

— Jack Espy, Athlete

What’s next

Espy will be using the Garmin Enduro 3 during the Leadville 100 ultramarathon in August 2026, as its significantly longer battery life of up to 320 hours with GPS is better suited for the demands of the 30-hour race.

The takeaway

While the Garmin Fenix 8 is an excellent all-around fitness tracker, its battery life limitations make it unsuitable for the extreme demands of an ultramarathon like the Leadville 100. Endurance athletes must carefully select gear that can reliably perform for the duration of their events, even in the face of extended use of GPS and other power-hungry features.