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Depends where the weight is going from and what it was.
Could be water, fat, muscle or sh.t.
One or two facts.
Runners fast fuel, glycogen is stored in muscles (and liver). For each gram of glycogen stored, two grams of water are stored. So if a diet is anti carbohydrate its essentially out to reduce water weight. No good for a runner.
If you don't train and starve a bit, and avoid drinking much you'll lose muscle mass and thereby water and weight and strength. So you'll be lighter but won't run better.
Bad case of the 'trots' will shift weight fast.
Losing fat but not muscle is much more difficult. 3600kcals in one pound of fat.
Its beyond most people to deal with the process of trimming fat off on account of the energy calculation. If anyone can take one pound per week off, then they are doing well.
Takes ages to permanently shift fat.