Unless you replenish fluids, just an hour’s hike in the heat or a 30-minute run might be enough to get mildly dehydrated, scientists say. Published July 30, 2018 at 4987 × 3740 in Off Your Mental Game? You Could Be Mildly Dehydrated Unless you replenish fluids, just an hour’s hike in the heat or a 30-minute run might be enough to get mildly dehydrated, scientists say.