For decades, marathon training has been governed by an unyielding law: the prescriptive training plan. Runners download a static 16-week calendar, print it out, and treat it like holy scripture. But life—in the form of late nights at the office, family obligations, or a sudden winter flu—invariably gets in the way. When an amateur runner misses a Wednesday interval session or a Saturday long run, the standard psychological response is guilt, panic, and the dangerous urge to “make up” the miles. A dynamic marathon training load could perhaps change this.
Josh Sambrook, a sports scientist and 2:28 marathoner, believes this entire framework is broken.
“For amateur athletes who have to fit their running around their life—rather than elite athletes who get to fit their life around their running—a prescriptive training plan doesn’t factor in the realities of a multifaceted life,” Sambrook explained on a recent episode of the Marathon Running Podcast.
His solution? Throw out the daily schedule and focus purely on cumulative, dynamic training load. Sambrook is the creator of Kaizen, an adaptive running app built to act less like a strict drill sergeant and more like Google Maps for your fitness.
Rerouting After a Wrong Turn
The core philosophy behind Kaizen—named after the Japanese business principle of continuous, granular improvement—is that individual workouts matter far less than consistent, progressive volume over time.
Instead of demanding a specific workout on a specific day, the app calculates a weekly “training load target.” It tracks the relationship between a runner’s distance, pace, and elevation, dynamically updating the remaining goals for the week based on what actually happens on the asphalt.
The system truly shines when life derails a training block. If a runner falls ill and misses three days of running, traditional plans leave them stranded. Sambrook’s algorithm behaves differently.
“When you take a wrong turn on a drive somewhere, Google Maps doesn’t attempt to necessarily route you back onto the same route. It doesn’t just throw its hands up and say, ‘Well, you can’t get to your destination now.’ It says, ‘Given the reality on the ground of where you’re at, what is the most practical, achievable way to get you to your destination?’ We’re doing exactly the same thing, but with fitness.”
The Unpopular Truth: Less Gym, More Jogging
Beyond the adaptive technology, Sambrook’s approach offers a refreshingly controversial take on endurance performance that may surprise runners accustomed to filling their weeks with track intervals, plyometric drills, and heavy gym sessions.
According to Sambrook, for the time-pressed amateur athlete, many of these elements are “one-percenters” that shouldn’t take precedence over basic volume.
“I am of the opinion—and I’m aware that this is a somewhat unpopular opinion—that I would rather spend that time and encourage people to spend that time going for just a jog and logging some easy miles rather than carving out precious time to do, for example, strides,” says Sambrook.
While he acknowledges that targeted strength interventions are vital for managing chronic injuries, his data suggests that generalized gym workouts do little to prevent new injuries in healthy runners. The ultimate shortcut to a faster race time? Running further, more consistently.
To prove his point, Sambrook points to elite data within his system, including a runner who recently shaved 90 seconds off an already blistering 1:07 half-marathon time. The breakthrough didn’t come from a new speed matrix or lifting weights; it came from adding 20 kilometers of easy volume to his weekly baseline.
Romance vs. Reality on the Start Line
For those who wonder how a system built on easy miles can prepare a runner for the grueling pace of a race day, Sambrook points back to the numbers. His algorithms, which have been in development for a decade, boast a staggering 93% accuracy rate in predicting road race finishing times to within 5% of the exact mark.
This predictive power effectively removes the anxiety and guesswork from race morning. Runners don’t have to guess their marathon pace based on a single fluke workout; they line up knowing precisely what their cumulative training load has earned.
“It can take a paradigm shift in your head,” Sambrook admits, recalling his own 2:28 PR where he did almost zero traditional speed workouts, relying instead on high-volume run-commuting. “But it works. When I line up for a marathon, I’m not questioning what time I’m gonna get. I am collecting the result that I’ve earned in training.”
For the everyday marathoner tired of playing catch-up with an unyielding training calendar, the message is clear: stop trying to be perfect, embrace the detours, and just keep moving.
The Kaizen app is available now on the Apple App Store and Google Play Store.
For a deeper visual dive into how these prediction models parse your running data alongside major platforms, check out this video breakdown on Predicting a 10K Time Using Kaizen and Strava. This clip demonstrates how the app evaluates cumulative training load to accurately project real-world race outcomes.
