I also like the Cliff Shot Blocks as well as real food. I have run a half but am primarily a triathlete. It does seem like blocks of more intense training will highlight weaknesses in my program. I am gluten and dairy free and trying to eliminate nightshades. I have huge issues with joint pain, muscle weakness, bloating/swelling, and fatigue that adversely affect my training.
I find a journal that tracks my food as well as how I feel and perform to be very helpful. I slacked off recently and had a bad flare up of symptoms. I realized how important it was when I couldn't identify any patterns or culprits.
Overall, the training has actually helped me identify and manage the issues as I am so tuned in to my body. Good luck with the marathon and don't forget to HAVE FUN!!