I did bugger all running last spring and did a fairly decent marathon off one long run of 13 miles. I had legs from the previous year though and I'd been doing about 6 bike or turbo or spin sessions a week.
good to hear, I've got one in 3 weeks off the same kind of training
Most ultras have cut off times (that they are too generous is another debate) most marathons do not (again with the ones that do, that they are too generous is another debate) Which makes completing an Ultra a challenge in a way that completing a marathon is not.
and a 17min 5k is much harder to achieve than a 3hr marathon.