Is your goal time realistic?
Most goal times are picked because they are round numbers. This works out what your current fitness is actually worth, how far your target is from it, and whether you have enough weeks to close the gap.
What the number means
The likelihood is not a probability in any strict sense — nobody can give you that. It is a read on the distance between where your fitness is today and where your target sits, adjusted for how long you have to close it.
Anything at 80% is a goal that matches your current fitness: turn up, stay healthy, execute on the day. At 65% you need the whole block to go well. At 40% you need it to go perfectly, and one bad month takes it away. Below that, you are not choosing a hard goal — you are choosing a goal that needs a different training cycle first.
Why your longest run may change nothing
Every build starts its long run at a floor set by the goal distance, not by you: 3 km for a 5K, 6.5 km for a 10K, 9 km for a half, 13 km for a marathon. If your current longest run is below that floor, the plan starts there anyway — so entering a smaller number will not shorten the build, and the calculator will say so.
Above the floor it matters a great deal. A runner already comfortable at 20 km needs far fewer weeks to reach marathon shape than one sitting at 10 km, and that shows up immediately in the minimum build length.
The long run is usually the real constraint
Runners underestimate weeks because they think about speed. Speed responds fast. Endurance does not, and it cannot be rushed: the long run climbs at roughly 0.5 km a week for a 5K build and 2.5 km for a marathon, with a recovery week every third or fourth week and two weeks of taper at the end. That arithmetic, not your ambition, sets the minimum.
A goal you miss teaches you less than a goal you hit
This is why the stepping stone exists. A block that ends with a personal best and a fitter runner is worth more than one that ends 4 minutes off a number you were always unlikely to reach. The ambitious time is still there next cycle, and you will arrive at it from a better place.
What this does not know
- Your training history. A 25:00 5K off six weeks of running and a 25:00 off six years are not the same fitness.
- The course. Hills, heat and wind are worth minutes, not seconds.
- Whether you will actually do the work. Adherence beats every model here.
The Outrunr app closes those gaps by scoring your goal continuously as you train, rather than once before you start.
Common questions
How is the likelihood worked out?
Your current 5K is projected to the goal distance using Riegel's formula. The gap between that projection and your target is then read off a fixed ladder: within 3% either way is achievable, 6% is a stretch, 10% is ambitious, and past 15% is beyond what one training block delivers. Longer plans get a modest upward adjustment, because more weeks means more adaptation.
Why does it say my plan is too short?
Because of the long run, not the speed. Your long run can only climb so fast without injury, and it has to reach a race-appropriate distance before the taper starts. If there are not enough weeks to get there, the warning fires and tells you what your long run would actually peak at.
What is a stepping-stone time?
When the likelihood drops below 45%, the calculator suggests a target about 65% of the way from your realistic prediction toward your goal. It is the time that makes this block a success and sets up the next one, instead of a block that ends in a time you were always unlikely to hit.
What is the warning about easy pace and race pace?
If your goal race pace is very close to your easy training pace, you will end up running almost every session near race effort. That is how runners get injured and burn out mid-block. The gap should be at least 15 s/km for a 5K goal and 45 s/km for a marathon.
Does this account for how much I train?
Only through the longest comfortable run and the number of weeks. It deliberately does not ask for weekly volume, because a single number for volume hides whether it is consistent — and consistency is what actually decides the outcome.
Is Riegel accurate for the marathon?
It is optimistic for the marathon, and honestly so. Riegel assumes you are equally trained for both distances, which almost nobody is when projecting from a 5K. Treat a marathon prediction as the ceiling your aerobic fitness allows, with endurance still to be built underneath it.
More free tools
A verdict is not a plan.
Outrunr builds the weeks that close the gap — long runs that climb at a survivable rate, sessions at paces derived from this same fitness, and a confidence score that moves every time you run.