I'll cosign this more than once. I was humbled by my first go at the marathon distance. I bonked big time, and in that 16-18 zone. I ignorantly believed I could run strong on an avg of 38 mpw across June-August. It was consistent but for my desired pace, I was wayyy off. It's such a demand on the mind and body. I finally understand why so many preach that if you want to run strong, a block of 60 mpw has to happen eventually. If your body and time allows it, getting to around 70+ mpw will be even better. All with good recovery though. The Half Marathon and Marathon are 2 different worlds.