Hello,
The way I view it is as follows: if one wants to be really good at something, one has to practice the craft per se. Running is no exception. Usually, the classic way to train is 70-80% of weekly volume in Z2. Roughly, the remaining portion is equally divided into interval work and sprint work (Z5). All this volume usually has to be done with running because running remains a skill.
If one do not like running, then, challenge aside, there is no reason to run an ultra. Almost everyone is able to run a 5K or 10k, assuming there is no previous injury or medical constraints. However not everyone can run a 10K in 30 minutes. Plus, being able to run a 10k without being injury during or afterwards does not mean one has good form. It may simply mean that one did not reach the threshold where poor form ends up in injury
GPP will obviously help, whether we use S&S as main tool or CF or whatever. For instance, will a military run / ruck faster and heavier than average ? Probably. Will he be able to lift more and more often than average ? Probably. However, will he be better than a professional runner ? Probably not.
Kind regards,
Pet'