I was planning to do a 80-90 min run today with a fartlek in the middle of 10x2 min @ 5k pace with 2.5 min easy. The weather was cool, cloudy, and windy. I felt good starting out in the beginning and thought it was going to be a great run. After 3 miles I began the workout and felt okay on the first 4. This was an out and back run, so I turned around there and it was very windy and really annoying. The next 2-3 were okay, and then on the 8th one I started to feel it. On the 9th one it felt like my legs were tying up (they were already hurting) and then I skipped the 10th one because I had to go up a hill and my legs were really burning and feeling it. I ran for another 15-20 minutes after that as a cooldown and my legs could really feel it and burning. Though I was sweating a ton, I also felt stiff and cold (like I said, I had a headwind going against me). I don't know what happened... my legs really hurt at the end when I finished. Anyone have any experience with this? I think I ate fine through the day so I don't know what happened.