Alex Yee, Olympic and World Triathlon Champion, is stepping into uncharted territory. This April, Yee will make his marathon debut on his home streets of London. As a master of three disciplines—swim, bike, and run—Alex Yee is emphasizing the latter as he aims to improve his already exceptional discipline of running.

Find out how he’s specifically preparing for the London Marathon and how a multi-discipline approach could help you build fitness for marathon success.

Alex Yee's products of choice: COROS Heart Rate Monitor & COROS PACE Pro

Software: COROS Training Hub

Why the London Marathon?

Coming off a standout year in 2024—claiming both the Olympic and World Championship titles—Alex Yee spoke about why the marathon makes sense now.

“The marathon is something I’ve wanted to try for a while. There’s something so enticing about the distance. Taking it on now just makes sense—it’s something different to mix things up and keep the motivation high. And what could be more motivating than running in my hometown?”

Although Alex is known for his exceptional running ability, he still sees room for improvement.

“I’m by no means a complete triathlete yet. I spent a lot of time working on areas I needed to improve for Paris. With a few years until LA 2028, I want to see how much I can progress across all aspects of triathlon. Since working with COROS, I’ve been able to analyze different areas in my data that can be improved. It’s an exciting time in my career.”

Building a Strong Foundation for the Marathon

As a triathlete, Alex is used to long training hours across multiple disciplines—an approach that aligns well with marathon preparation. While swimming and cycling remain part of his weekly routine, they have been scaled back in favor of run-specific training.

This cross-training approach has helped Alex steadily build his aerobic base, proving that marathon training doesn’t always have to follow a conventional path. His goal is to continue increasing his Base Fitness until he begins tapering, ensuring he’s primed for peak performance.

“The start of 2025 has been a little up and down, but being able to see my data moving in the right direction is reassuring. I know my body and what it responds well to, and seeing those improvements in the numbers reinforces that I’m on the right track.”

COROS Education: Base Fitness is a 42-day average of your training load, reflecting your aerobic fitness. It helps athletes gauge the progress of their training over time.

The Role of Heart Rate in Marathon Training

Elite marathoners are conditioned to race around their threshold pace, making heart rate (HR) a key metric in Alex’s preparation. He closely monitors his HR during key sessions to ensure he’s training efficiently.

“Heart rate is a great metric to track across all aspects of my training. Alongside how I feel, it helps me understand how my body is coping with the workload. The COROS Heart Rate Monitor has made this tracking much more reliable.”

Alex’s heart rate threshold sits between 172-184bpm, allowing him to sustain around a 3:02/km pace. This translates to a potential marathon finish of around 2:08—a target that aligns with what he believes he’s capable of achieving.

"I'd love to run somewhere around 2:07 to 2:10. We are going into the unknown, so we will see, but I want to give it a really good go."

Alex's Key 30km Marathon Session

A key part to Alex's training is his marathon-specific long run. One of the most intense sessions in his buildup has been a 30km progression run at an average pace of 3:07/km.

“I’m really enjoying these longer sessions. Going from sessions like 6 x 2km as my max volume to now running 30km continuously is a big jump, but my body is responding well—and most importantly, I’m loving the challenge.”

The goal of this session was to replicate race-day demands. Starting at 09:35—matching the official London Marathon start time—Alex eased into the effort, covering the first 5km in 17:21, at a steady heart rate of 143bpm. Progressing into the bulk of session, the next 20km were completed at an average of 3:04/km, finishing up this portion of the workout in his threshold HR zone at 172bpm.

With 25km in his legs, Alex pushed on for the final 5km towards the lower end of his threshold pace, averaging 2:58/km, finishing his 5k in 14:50, keeping well within his threshold zone with an average 177bpm heart rate. This training approach helps him prepare for responding to race-day surges and mentally tackling the tough final miles of a marathon.

COROS Education: Progression runs are an effective training tool to help you build endurance, develop pacing skills, and improve your mental toughness for race day.

Tips from Alex for any Marathoner

Alex’s approach to marathon training provides valuable insights for runners of all levels. Here are the key lessons you can take from his journey:

1. Fitness can be built across multiple activities – Incorporating, cycling, or other forms of cross-training can enhance overall endurance.
2. Accurate heart rate data helps gauge effort – Monitoring HR ensures you’re training at the right intensity.
3. Train specifically for race-day demands – Long runs and marathon-pace sessions help prepare both the body and mind.
4. Set new challenges to maintain motivation – Mixing things up can keep you engaged while building toward long-term goals.

By using the right tools and following a structured plan, you can set yourself up for marathon success. Discover how COROS products and training plans can support your running journey!