I get a version of this question every fall. A runner is two weeks from Chicago or five from New York, feels good, and wants to know if they should add a second tempo run to lock in the fitness. The instinct makes sense. It is also backwards.
Threshold running feels productive. It is hard enough to notice, structured enough to log, and close enough to race effort that more of it seems like it should mean more race fitness. When you look at how well-trained endurance athletes actually train, that is not what the data shows.
Threshold work is real and it matters. It is also the smallest slice of a good plan, not the biggest one. I have coached and run enough marathon cycles to know how tempting the math feels: if one threshold run built fitness, a second one should build more. The research on how well-trained endurance athletes actually distribute their effort does not back that up.
The myth, and what training logs actually show
The exercise physiologist most associated with this question is Stephen Seiler, who spent years pulling training logs from rowers, cyclists, cross-country skiers, and runners to see how the best endurance athletes in the world actually distribute their effort. Thomas Stoggl and Billy Sperlich summarized that body of work in a 2015 Frontiers in Physiology review: across well-trained and elite endurance athletes, the typical split runs roughly 75 to 80 percent low intensity, about 5 percent threshold, and 15 to 20 percent high intensity.
Read that again. Threshold, the pace most runners think of as the meat of a build, is the thinnest slice on the plate. The bulk of the work is easy. A meaningful chunk is genuinely hard. Threshold sits in a narrow band between them, and it is not where the volume goes.
This pattern is called polarized training, and it is not just a description of what elite athletes happen to do. A 2024 systematic review by Henrik Lyngstad Nost, Morten Aune, and Roland van den Tillaar, published in Sports, pooled multiple randomized controlled trials comparing training approaches head to head. Programs built on a polarized distribution produced larger gains in VO2max and running economy than programs that leaned heavily on threshold-intensity work. It is not only what good athletes do. It is what worked better when researchers tested it directly.
What threshold pace is actually training
To see why more of it does not simply stack, it helps to know what the pace represents.
Lactate threshold is the fastest pace you can hold where lactate production and clearance stay roughly in balance. Push past it and lactate accumulates faster than your body can clear it, hydrogen ions build up, and the muscle environment turns acidic enough that fatigue arrives on a short clock. Threshold training raises that ceiling by training the clearance side of the equation.
Two transporters do most of that work. MCT1 moves lactate into slow-twitch muscle fibers and mitochondria, where it gets oxidized as fuel. MCT4 moves lactate out of the fast-twitch fibers producing it. Steady, sustained efforts at or near threshold train the first pathway. Short, hard, repeated efforts with brief recovery train the second, by forcing rapid lactate mobilization between reps.
Meanwhile, most of the underlying aerobic engine, mitochondrial density, capillary growth around the working muscle, and the fat-burning capacity that spares glycogen at easy paces, gets built by consistent low-intensity volume and genuinely hard intervals, not by the pace in between. Threshold work refines an engine that easy mileage and hard efforts mostly build. That is the physiological reason it does not scale the way easy volume does. The adaptation it targets is specific, and once you have trained it adequately, adding more threshold time does not compound the way adding another easy long run does.
What happens when you add more anyway
The instinct to add a second threshold session is not irrational. It just misreads which lever is available to pull.
In the trials the 2024 review pooled, the programs that leaned hardest on threshold-intensity work were not the ones producing the biggest fitness gains. Extra time spent at threshold pace has to come from somewhere in a finite training week, and it most often eats into either easy mileage, which builds the aerobic base threshold work depends on, or the quality of the genuinely hard sessions, because threshold running is fatiguing enough to blunt the next day's intervals without being hard enough to drive the same adaptation.
None of this means threshold running is a mistake. It means it is a small, specific tool, and using more of it does not make it a bigger one.
What this looks like two weeks from Chicago
One threshold-focused session a week, something like a sustained tempo effort or a set of cruise intervals at a pace you could hold for about an hour, is a full dose for almost every marathoner already running consistent mileage. In practical terms that is usually 20 to 40 minutes of combined threshold running inside an hour-long workout, not the whole session. That roughly matches the 5 percent figure from the training-log research, once you account for a recreational runner's total weekly volume rather than an elite athlete's.
With Chicago about two weeks away and New York around five, a threshold session added on top of an already-built plan does not have time to convert into fitness before race day. Physiological adaptations from a single extra hard session take longer than two weeks to show up in performance. What that session does have time to do is leave your legs tired for the easy mileage and goal-pace work that still matter more right now.
If you already have one threshold day in the plan, that is the dose. Protect it, run it well, and let the rest of the week stay easy. The fitness for Chicago or New York is already largely banked. What is left to manage is fatigue, not volume.
The honest caveats
Two things are worth saying plainly, because they get lost when a training percentage becomes a rule.
First, most of this research comes from elite and well-trained endurance athletes training many hours a week: rowers, cross-country skiers, cyclists, competitive runners. Direct evidence in recreational marathoners training four to six hours a week is thinner. The direction of the finding, easy volume dominates and threshold stays a small slice, likely holds. The exact percentages are a reasonable guide, not a law.
Second, a tempo run at goal marathon pace does something a pure physiology argument does not capture: it is a rehearsal. Practicing the pace, the fueling, and the mental discipline of holding an effort matters, and that value is real even though it comes from experience rather than from the same body of research. One well-placed session can do both jobs. It does not need a second one to do either better.
My wife Aubrie has run a 2:59 marathon and finished more than ten of them. Her hardest training weeks were never the ones with the most threshold volume. They were the ones where the easy days stayed easy enough that the one hard session each week actually meant something.
Where NXT RUN comes in
The hard part is not knowing that one threshold session is enough. It is knowing what pace that session should actually be at, week to week, as fitness shifts across a build.
Adaptive Pacing keeps your threshold and easy-day paces tied to how your actual sessions go, not a number fixed in week one. If a threshold run feels harder or easier than it should, future sessions adjust from there. The plan reflects the dose you are supposed to be running, instead of leaving you to guess whether a session that felt good means you are ready for more.
This article draws on training-intensity-distribution research conducted mostly in elite and well-trained endurance athletes. If you are newer to structured training, coming back from injury, or unsure how your body responds to hard efforts, the right amount of threshold work is smaller than what is described here and worth discussing with a coach familiar with your history.