Before you stop drinking water...

NEJM says:

Conclusions Hyponatremia occurs in a substantial fraction of (1) nonelite (2)marathon runners and can be severe. Considerable weight gain while running, a (3)long racing time, and (4) body-mass-index extremes were associated with hyponatremia, whereas female sex, composition of fluids ingested, and use of nonsteroidal antiinflammatory drugs were not.

If all these things apply to you and you drink more than 0.75 gallons per race, you might want to cut down. But as Mickey was arguing to me, it is probably more dangerous to stop drinking than it is to be careful (as hyponatremia only occured in 0.6% of their sample, i.e., 3 people out of 430).