Is my ideal marathon pace always 15 seconds per mile faster than my MAF pace?

*warning: this is a highly technical answer*

Yes.

Or put another way, you never assume otherwise unless you have laboratory evidence that says otherwise. That’s because the more fat-adapted runner would be burning a greater percentage of fats at any given time, but he would also be burning energy OVERALL at a much higher rate, which means that the more fat adapted runner will be burning as much sugar, because that highly trained runner is exceeding their MAF HR by a similar amount. This means that the same amount of sugar is equivalent to a smaller percentage of overall fuel consumption. Here’s a great research article with corroborating evidence.

Think about it this way: if running 15 seconds faster than MAF means that in some time period X, you are burning 1 unit of sugar and 9 units of fat (10% sugar and 90% fat), becoming more fat adapted you can be burning 1 unit of sugar and 14 units of fat (7% sugar and 93% fat). You are still running 15 seconds faster than your MAF, but your MAF might be 45 seconds faster than before. So, in our little thought experiment, that 1 unit of sugar is what produces that 15 sec faster pace.

Now let’s apply it to the real world.

A 5 minute MAF miler can run MAF at 137% the speed of a 7 minute MAF miler (12 mph to 8.6 mph). By running 15 seconds/mile above their MAF HR, the 7 minute miler is running at 101% of their MAF speed. Conversely, a 5 minute MAF miler is running at 105% of their MAF speed. (A 15 second difference turns 12 MPH into 12.6 MPH, but turns 8.6 MPH into only 8.9 MPH. For comparison, 15 seconds turns 60 MPH into 75 MPH.)

So there’s another phenomenon here, which makes the real world different from the thought experiment above: to account for this speed increase, a runner running that marathon world-record 4:45 pace of 12.6 MPH—World-record holder Dennis Kimetto averaged 4:47 in Berlin—would be burning sugar at a greater rate than that 7 minute MAF miler, and their rate of fat-burning would be high enough that the rate at which they burn sugar would still mean that sugar burning accounts for a smaller percentage of total fuel consumption. In other words, the 7 minute MAF miler might be burning 1 unit of sugar and 9 units of fat (10% sugar and 90% fat) but the 5 minute miler might be burning 1.5 units of sugar and 18.5 units of fat (7.5% sugar and 92.5% fat). Even though it accounts for a smaller percentage, this theoretical world-record holder is burning sugar at 1 1/2 times the rate of the 7 minute MAF miler.

So, for a faster MAF runner to run 15 seconds faster than their MAF HR, their sugar-burning systems, which provide increasing energy above their MAF HR, have to provide energy at a greater rate than for a slower MAF runner to run 15 seconds faster than their MAF HR. Faster MAF runners also have livers with a greater ability to provide glucose, which means that it all evens out: the faster MAF runner ends up depleting their glycogen supply at a similar rate than a slower MAF runner, when both are running 15 seconds faster than their MAF. We’ve got a lot of observational data to corroborate this. In conclusion, adding 15 seconds to MAF time is a calculation that seems to work for a vast majority of runners across a huge range of speeds.