In general, a generator that's large enough to generate enough resistance to replace your brakes for relatively gentle stopping (you'll use your existing brakes, non-regeneratively, for more forceful braking) will be as large as a motor used to power a bicycle, and unless you can make some sort of transmission that can completely disengage when not used, it'll slow things down even when not activated. And then you'll need a battery pack large enough to accept 250 watts of charging power for a few seconds here and there.

This is a more complicated issue than you may have considered. It makes sense with an electric bike as the motor can generally be turned into a generator through circuitry, and it already has the battery pack ... but regenerative braking makes far less sense on a non-electric bike.

A standard bicycle generator setup charging a modest battery all the time with a few watts of power would be a lot simpler. Or skip the battery entirely and have the generator power your lights directly, perhaps with a capacitor to allow the lights to remain on for a minute or two after you stop moving.