Training works best when it is planned, not guessed. Many runners and active people start with energy, do too much in the first week, and then feel sore, tired, or bored by week three. The Half Marathon Plan Builder was built to stop that pattern. It turns a vague goal into a clear weekly plan you can follow without thinking too hard. You do not need an account, and you do not need to install anything. Open the tool, enter a few details, and get a clear result in seconds. That simple step, done at home, makes the next weeks easier, because you always know what today asks of you and what it does not.
This tool works in your browser. Enter how many weeks you have and how many sessions per week you can do. The builder grows your long run toward twenty kilometres, with a lighter cutback week every fourth week. It grows the long run by about 1.2 kilometres each week, then cuts it back to eighty percent every fourth week, so the body absorbs the training before the next build. The result appears below the form, in plain lines you can read at a glance. You can change any value and run it again, so the plan improves as your fitness or your schedule changes. Print the result, or copy it into your notes, and take it with you. The tool never asks for personal details, and nothing you type leaves your device, because all the maths runs on the page itself. This tool gives general training information only. It is not medical advice, and it does not diagnose or treat any condition.
Why does a planned build matter so much? Because fitness grows from steady stress plus rest, not from one heroic week. A plan that jumps too fast asks tendons, bones, and muscles to adapt faster than they can. A plan that never grows asks nothing, so nothing changes. The middle path is a small weekly increase, an easy day after a hard day, and a lighter week now and then. When you follow that shape, a hard session stays hard in a good way, and an easy session stays easy, which is exactly what lets you train again the next day.
How to use it, step by step. First, be honest about where you start: your current level, your free days, or your starting distance, as the tool asks. Second, enter those values in the form, using the units shown beside each box. Third, press the button and read the result from top to bottom. Fourth, change one thing at a time if the result looks too hard or too easy, and run it again. A good plan usually needs two or three short tries, and that is normal. The goal is not a perfect plan. The goal is a plan you will actually follow for weeks.
Start with the example already in the form. It shows a sensible starting point, with realistic numbers and a shape you can trust. Run it once, just to see the form of the result. Then replace the example with your own numbers, and run it again. Compare the two results. If your own plan is much harder than the example, that is a warning sign, not a challenge. Scale one number back, and the whole plan will feel calmer at once. Small plans finished beat big plans abandoned, every single time.
Respect the cutback week. It looks too easy on paper, and that is the point: fitness grows during recovery, not during the hardest week itself. Keep notes on how each week felt, in one line per week. Felt strong, felt tired, slept badly, or felt easy are enough. After four weeks, those notes show your real pattern better than any single number. If two weeks in a row felt hard, hold the plan steady for one more week instead of pushing on. If two weeks felt easy, raise one number slightly. Your notes turn a generic plan into your plan, and that is when progress gets steady.
Can I use this tool for free? Yes, it is free, with no account and no sign-up. Does it work on a phone? Yes, it works in any modern browser, on a phone or a computer. Can I change the plan later? Yes, change any value and run it again as often as you like. What if a week feels too hard? Repeat that week, cut one session, or lower one number, then carry on. The tool shows the maths clearly, so you can see exactly how each week is built. Remember, this is general education only, not personal medical or coaching advice.
Spanish version: Constructor de Plan de Media Maraton