98 Mustang Coolent Issue! Help!

I just recently purchased a 98 Mustang v6. I decided to change the coolent because the car has 70k on it and it looked dark. I drained everything out, and cleaned out the fill tank. I then added new coolent to the fill tand up to the Fill hot line as my haynes book states. I drove the Mustang a couple miles down the road and noticed the coolent gauge is running all the way hot. I wondering what I could have done wrong to cause this. Please Help! I need to drive my car.

Thanks
Greg

well, if the fill tank was the only thing you put back in, and didn't get the radiator full, yeah, it's gonna run hot...