The longer I do this, the more I realize it all comes down to eating. If your legs feel shot, it means you need to eat. On last week's 200k, my legs felt shot and crampy, and we stopped to eat. After that, I felt great, my legs felt great. I thought I had been eating enough, but apparently not. On a randonneuring 400k, we almost always have an opportunity to eat real food, but I am not sure on a double. Keep in mind that you will be riding when you normally eat lunch and dinner. You may not feel like eating at times, ignore that and eat anyway.