Importance of hydration - lesson learned

All;
I was out on my LSD run on sunday morning. Things were going along quite well. I started just before sunrise. It was a lovely morning. The sun came up and things heated up. I was hydrating adequately (or so I thought) I got to 12 1/2 miles and my legs tightened up, running became painful. By 14 miles I could barely walk. Luckily this coincided with my house. Climbing the steps to the front door was another matter however. Just lifting my legs off the ground was painful. Much of the rest of the day was spent resting in front of the TV.
Later I compared my fluid intake with my marathon coach, who was running at a similar time to me. He had take 70 fl.oz for 20 miles. I had taken 20 fl. oz to 12 miles! No wonder I was in trouble.
I won't make that mistake again.

ian