Today was miserable

The marathon training guide I am following had me run 4 miles Tuesday, Wednesday, and Thursday. I had a rest day on Friday and for today I had to run 3 miles. I figured this would be an easy three miles... boy was I wrong. 42 min later I finally got off the treadmill feeling like junk. Both my legs are cramped and I felt exhausted. To make matters worse my training guide has me running 5 miles tomorrow. I truly don't know how I am going to run that tomorrow. Well I guess the only thing I can do is rest up and hope for the best tomorrow.