Our expert says:
No, you wouldn't be losing water - your body would defend the total volume quite tightly, so you don't lose water over prolonged periods. There's a better chance that you're losing fat and building some muscle, which is a normal and very positive change on training programmes. It's difficult to say, because I know nothing of what you've done so far, but it would be relatively common for this to happen after training for a while.
The key is that you're also getting fitter - if you are, then give it a little more time and you'll start seeing results too. The goal should therefore be to run, cycle, swim (whaterver you do) further or faster while feeling the same. If you can do that, then the result will take care of itself.
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