Half marathon training

Here's my question, with some context. I was told its a good idea to weigh one's self before and after a long run to ensure one is properly rehydrated (drink an amount of water equal to the weight loss). The other day though-- I lost 3 lbs on a run!! That's mucho agua. So, here's my question-- is that even possible? I get that some of the sweat evaporates, but most of it stays on clothes and skin right? I could become a humidifier if I'm actually releasing that much steam in an hour. I'm so perplexed by this whole thing.