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Not much is the answer.
Let’s say you are middle aged and already in a healthy weight zone. Let’s also say that you ran 5k in 20 minutes (I only give that very respectable figure so that you can see the percentage improvement more easily).
To knock off 20 seconds and improve your time to 19:40 OTHER THINGS BEING EQUAL you would have to lose about 10% of your body weight ie lose 7kg if you weigh 70kg. That is a lot of weight to lose for a mere 1.5% improvement. Although it is a notable improvement nevertheless.