It takes time and I encourage runners of any level to start thinking about running, if they like it, in yearlong blocks with annual goals. My longest race to day is a 30 mile trail ultra. Ive done a variety of sports all my life so the fitness base was there. Then I committed to specifically training for that distance. Truth be told - as long as your training is consistent, intentional and you want it, eating is the hardest part. By far. 100k is a different beast altogether. It takes a community to tackle 24-30 hours of ruunning!