About the Long Run Progression
This calculator plans a week-by-week buildup from a runner's current long run distance to a target distance using the widely cited 10% rule. Enter your current and target long run mileage to see how many weeks the progression takes and what distance to run each week along the way.
How It Works
The calculator increases the current long run by 10% each week until it reaches the target distance, capping the final week's distance at the target itself so the progression never overshoots. It also calculates the number of weeks needed algebraically using logarithms, then builds a week-by-week table showing the distance for each week of the plan.
Formula & Methodology
For a current long run of 8 miles building to a target of 20 miles, the weeks-needed calculation is the log of (20 divided by 8) divided by the log of 1.1, which works out to roughly 9.7, rounded up to 10 weeks. Building the table week by week: week 1 is 8 x 1.1 = 8.8 miles, week 2 is 8.8 x 1.1 = 9.7 miles, and so on, compounding by 10% each week until the distance would exceed 20 miles, at which point the calculator caps that week's value at exactly 20.
Examples
Half marathon to marathon buildup
A runner currently doing an 8-mile long run wants to reach a 20-mile long run. The calculator returns 10 weeks, with week 1 at 8.8 miles, climbing week over week, and the final week capped at exactly 20 miles.
A shorter progression
A runner moving from a 10-mile long run to a 13-mile long run needs roughly log(13/10) divided by log(1.1), or about 2.8 weeks, rounded up to 3 weeks, with the last week capped at 13 miles even though 10% compounding would slightly exceed it.
Advantages
- Automates the compounding 10% calculation across every week of the buildup instead of requiring manual multiplication week after week.
- Caps the final week's distance exactly at the target, avoiding an unintended overshoot that a naive multiply-by-1.1 loop would produce.
- Gives an upfront total-weeks estimate using the logarithmic shortcut, so a runner can see the full timeline before committing to the plan.
Common Mistakes
- Treating the output as a rigid mandatory schedule rather than a guideline, when real training plans typically include periodic down weeks the calculator does not model.
- Entering a target distance that is not actually greater than the current long run, which the calculator rejects as invalid input.
- Assuming the 10% rule is a proven injury-prevention guarantee rather than a conservative convention with mixed research support.
Edge Cases to Watch For
- The calculator returns an error if the current long run is zero or negative, or if the target is not strictly greater than the current distance, since the logarithmic formula and the progression both require a positive starting point and forward progress.
- The final week in the table is capped at the target distance even if a straight 10% increase from the prior week would have exceeded it, so the last increment is often smaller than 10%.
- The plan does not build in recovery or 'down' weeks, so it represents a continuous compounding increase rather than the periodized schedule many real training plans use.
Common Use Cases
- Runners building from a shorter race distance toward a half marathon or marathon long run.
- Coaches sketching a rough weekly mileage progression before layering in speed work and recovery weeks.
- Return-to-running athletes rebuilding long run distance gradually after time off.