Most likely you didn't drink enough water and didn't eat enough. Pretty simple really. Your body can only store about 2000-2500 calories of glycogen. That's good for about 30-45 miles. Supplement that with about 800-1000 calories of fat-metabolism and the furthest you're gonna go is about 60-65 miles before you bonk at best. To do a century safely, you gotta drink at least six 20oz bottles of water and ingest about 2500-3500 calories in energy drinks and food.