How to set a realistic marathon goal time
Most marathon goals are chosen because the number is round. A better method is to start from a race you have actually run and convert it, then adjust for the things a formula cannot see.
Why round numbers are a trap
Sub-four, sub-three-thirty and sub-three are appealing because they are memorable, not because they reflect anyone in particular. If your fitness says 4:07, chasing 3:59 means running the first half about eight seconds per kilometre too fast, which is exactly the margin that turns the last 10 km into a walk.
The goal should come out of the arithmetic, not be imposed on it.
Convert from a race you have run
The standard method is the Riegel formula, which predicts one distance from another:
predicted time = known time x (new distance / known distance) ^ 1.06
The 1.06 exponent is what accounts for slowing down as distance grows. Some coaches use 1.07 or 1.08 for the marathon specifically, which gives a more conservative and often more accurate prediction for anyone who is not highly trained.
A worked example
Say you ran a half marathon in 1:45:00, which is 6300 seconds over 21.0975 km.
The ratio of distances is 42.195 / 21.0975, which is exactly 2. Two to the power of 1.06 is about 2.085. So 6300 x 2.085 gives roughly 13,135 seconds, or about 3:38:55.
The quicker shorthand version, doubling your half and adding ten to twenty minutes, gives 3:40 to 3:50 from the same input. Both are in the same territory, and the shorthand is more honest about the uncertainty.
Adjustments the formula cannot make
Riegel assumes you are equally trained for both distances. That is rarely true, so adjust:
- First marathon: add ten to fifteen minutes. The formula does not know that you have never been past 32 km.
- Low weekly volume: if your long run peaked below 28 km, add more. Endurance is the limiter, not speed.
- Heat: expect to lose two to four percent in warm conditions, and more if you are not acclimatised.
- Hills: a rolling course costs more than the elevation profile suggests, because the downhills do not give back what the climbs take.
- Old result: a race from six months ago describes six-month-old fitness.
If you have no recent race
Plenty of first-time marathoners have never raced anything, which leaves the formula with nothing to work from. Two options are better than guessing.
The first is to go and race something. A parkrun or a local 10K six to eight weeks out costs you one weekend and replaces speculation with a number. It is comfortably the most valuable session in the block.
The second is to use your long runs. If you can complete 30 km at a genuinely comfortable, conversational effort, your marathon pace is somewhere near that effort, not faster. Take the average pace of your best long run and add a little for the extra 12 km. It is cruder than a race result but it is grounded in something you actually did.
What does not work is picking a pace from a training plan written for someone else, or from what a friend of similar build ran. Neither has any information about you in it.
Sanity check the pace, not the time
A finish time is abstract. The pace it demands is not. Convert your target into minutes per kilometre or mile and ask an honest question: could you hold that for a two hour training run today, and still feel like there was more?
If the answer is no, the goal is too fast. Marathon pace should feel comfortable and slightly boring for the first two hours. If it feels like work at 10 km, it is the wrong number.
Set two targets, not one
Pick an A goal for a good day and a B goal you are confident of on an ordinary one, and start at B pace. If you reach 30 km feeling strong, the A goal is still available. If conditions turn or the day is simply not there, you have a race you can still finish well rather than a target you abandoned at halfway.
Starting at A pace gives you one outcome if it goes right and no outcome at all if it does not.
Put a number on it
Work out the pace any target demands, with split times you can carry into the race.