How RunReady builds your plan

Step 1: Predict what you could run today

RunReady starts from one recent run where you held a genuine, steady hard effort. From that single result it estimates what you could run at your goal distance right now, using the Riegel formula:

The formula comes from Peter Riegel, an engineer and marathoner who published it in the late 1970s after studying world-record times across many distances. The key idea is the exponent. If running pace held constant, doubling the distance would exactly double the time (an exponent of 1.0). In practice everyone slows down as races get longer, and an exponent of about 1.06 captures that slowdown surprisingly well across a wide range of distances.

Example: a 25:00 5K predicts roughly 1:55:00 for a half marathon and roughly 3:59:46 for a marathon.

Step 2: Rate how realistic your goal is

Next, RunReady compares the prediction to your goal time and works out how much faster you would need to get, as a percentage. It then estimates how much improvement is plausible in the time you have, based on your experience level:

• Beginners start with the most room to improve, about 1.3% per week, because early fitness gains come quickly.
• Intermediate runners start around 0.7% per week.
• Advanced runners start around 0.4% per week, since they are already closer to their ceiling.

Those weekly rates shrink a little each week (diminishing returns), and they compound over the days until your event. The ratio between the improvement you need and the improvement the model thinks is available gives your rating:

Step 3: Set your training paces

Your paces come from the same recent effort, not from your goal. That is deliberate: training at the pace you hope to run, rather than the pace you can run, is a common route to overreaching. RunReady predicts your current 5K, 10K and half-marathon paces, then sets ranges around them. For a runner with a 25:00 5K, that works out to roughly:

Tempo and speed days include warm-up and cool-down running, so the workout portion is only part of the session.

Step 4: Pick your running days

You tell RunReady how many hours you could run on each day of the week. It then chooses how many days to use: three for beginners, four for intermediate runners and five for advanced runners (or fewer if you have fewer days free). The long run goes on the day with the most time, with a preference for your race day of the week and for weekends. The remaining days are spread out so hard sessions are not stacked together.

Intermediate and advanced runners with at least four days get a weekly tempo run; advanced runners with five days also get a speed session. Any easy day that falls right next to a long run or workout becomes a slower recovery run.

Step 5: Build the weeks

• Starting point. Week 1 starts from your recent weekly distance, not your goal.
• Gradual build. Weekly distance rises about 5% per week until it reaches your desired peak, or the most your available hours can hold.
• Cutback weeks. Every fourth week drops to about 80% of the current build so your body can absorb the work.
• Long-run limits. The long run is 30–40% of the week (depending on how many days you run), never grows more than 10% from one long run to the next, is capped at three hours of running, and has a ceiling for each event (for example, about 20 miles for a marathon).
• Taper. Half-marathon and marathon plans have a taper week at about 70% of peak volume. Race week keeps only short, easy running, about 20% of peak, plus the race itself.

If your available hours cannot hold the target distance, the plan tells you about the shortfall instead of quietly squeezing in more running.

How accurate is the Riegel formula?

For a quick estimate from a single run, it is one of the most useful tools available, and it is widely used in running calculators. But it rests on assumptions that are not always true for you:

• It assumes you are equally trained for both distances. A fast 5K says a lot about your speed and less about your endurance. Runners who do little weekly mileage often find that predictions for the half marathon and especially the marathon are too optimistic.
• The further apart the distances, the bigger the error. Predicting a 10K from a 5K is more reliable than predicting a marathon from a mile. If you can, use a recent effort that is closer to your goal distance.
• The input has to be a real effort. An easy jog underestimates you; a downhill or wind-aided run overestimates you.
• Conditions matter. Heat, humidity, hills and altitude can slow you well beyond what any formula predicts.

Treat the prediction and rating as a starting point for a conversation with yourself, not a verdict. If your training is going well, re-enter a new effort every few weeks and let the plan update.

What RunReady does not do

It does not know your injury history, sleep, stress, terrain or how your body responds to training. It cannot adjust from day to day the way a coach can. If something hurts, rest and get it looked at; no plan is worth training through pain.

Not medical advice. This page is general educational information for healthy adult runners. Talk to a doctor before starting a new training program if you have a health condition, and stop and get checked out if a run causes pain.