Sometimes I'm really stupid

A 12 mile run on the hottest day of the year so far finishing just about noon - not my best plan yet. I did stop at 12 miles at my friends house who kindly gave me water (did I mention that I didn't hydrate at all and didn't take any fluids with me). I then got really dizzy and had to sit on the floor for a bit. I've just forced myself to drink 500ml of electrolyte mix which is gross but should help as should the banana I've just guzzled.