Persistant Drymouth after Exercise?

I'm a fairly out of shape 17 year old kid. But yesterday, I began my journey on weight loss and getting in shape. After walking almost 4.5mi yesterday, I was sweating like crazy. I showered and I still kept sweating! My problem however, is that last night, about 7 hours after walking, I began to experience some thirst. 12 hours and maybe a a liter of water later, I'm still really thirsty o.o Drinking water temporarily relieves it, but nothing long term. Please help me! I'm scared T_T