Running a 3:20 km would result in a 2:20:39 marathon time (assuming you maintained that pace for the entire race).
Of course, simply multiplying your one km pace by 42.195 to determine your marathon time is unlikely to yield an accurate result because it doesn't account for fatigue that occurs as your distance increases.
To get around this limitation, Peter Riegel famously came up with a formula that factors in fatigue over longer distances: T2 = T1 x (D2/D1)1.06, where T1 is the time you know multiplied by the distance you're going to run divided by the distance you ran to a power of 1.06. In this instance, if you ran one km at a 3:20 pace, the formula would look like this: 3:20 x (42.195/1)1.06, for a result of 2:56:03. As you can see, this is significantly different than a straight extrapolation.
However, many people say Riegel's formula is outdated and based on the limited data he had at the time, and a newer formula that uses a power of 1.15 may be more accurate. Using that formula, the marathon time based on a one-km pace of 3:20 would be 4:06:33.
Again, these calculations are most accurate when running a singular km, not using an average km pace for a longer race duration, and estimating your marathon time from there.
When training for a marathon, you're likely not running a full marathon each time you go out and run. However, even if you're running shorter distances as part of your training program, you can take your average pace per km and project what your full marathon time would be. This will give you a good benchmark, especially if you track it over time, and can help you train even better.
Keep in mind that even if you can maintain a pace of 3:20 per km for one segment, that doesn't necessarily mean you can maintain a 3:20 per km pace for a full marathon (42.195 km) and achieve a marathon time of 2:20:39. Running progress takes time, and it's not uncommon for someone to be able to maintain a pace for an early part of a race but not later stages, especially if they're just starting out.
|Faster pace||Marathon time|
|Slower pace||Marathon time|