Marathon Training Week 12

I am on my 12th week of training for my marathon in September. This past weekend I ran 12 miles, it was really hard. I even had to stop and walk for ten minutes because my leg cramped up. That was the first time I have ever had to stop while running but I figure that it is okay because I don't want to injure myself. My longer runs are getting harder and harder and I need to start taking supplements while I run so I can keep my energy up. Training for a marathon is really time consuming and I hope I can keep up.