Answer - Train for the longest distance you have time and energy for; but race the shortest distances you enjoy.
Training for longer distances gets you fitter; and when you race the shorter distances you are testing your fitness levels without having the extra stress that comes with a longer race.
Your shorter race times will improve as a result of the longer training. Guaranteed.
The Time Predictor below is pretty good if people are trained up for the longest distances - but building up to the workloads involved takes years and is probably only possible for the younger runners.......
I raced at a high level for 13 years before I ran my first marathon .. I was 30 when I ran my pb but had been running for 17 years then.
Deek raced and trained hard for ten years before he ran his first marathon, and look what he became.
I advise people to do marathon training if they want to, but to race shorter distances and avoid marathon races until they have achieved their shorter distance targets.
Then the Time Predictor works really well.
Because if you a fit for a particular distance (eg 6k), then you are also fit for all shorter distances (eg 3k to 5k). But not necessarily for any longer ones (eg 10k and up).
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