For a few years I participated in the National 24 hour challenge in Michigan, as well as doing rando rides. I targeted 400 miles, and got there after 4 years. Then I quit ultras.
It was a learning experience, but my main takeaway is that I enjoy what serious ultra riders call garbage miles - LSD. I live for garbage miles. I train so the garbage miles will be more fun. A great 1200 for me is one where I ride quickly, do non-sleep controls quickly, sleep lots, and stop at every bakery and roadside stop that appeals to me at the moment. Lon Haldeman's advice sounds like a tolerable training strategy to me. I'm probably never going to be a numbers guy.