Hi can a beginner with sub 7.30m/km run finish a maratron?
Dont see why not as far as aerobically is concerned, but bro it's more than fitness.
Coz also have to practice things like nutrition for eg, some people do throw up when heat stressed, cannot stomach any fuel, a rare few might not even take in 100 plus or water. Vicious cycle then, leading to DNF.
And running that slow = exposure to the sun past 9.30am possibly (if not cloudy day)....... last year during stanchart that would be on WCH. Major fugfest i heard.....ambulances etc. So heat training.
Remember, the slower you run, the worse it gets, so another spiral down kinda vicious cycle in which those not so well trained lagi have it worse and worse.
If you are already at tempo at around 7+ mins pace (5hr+ timing as per pacing but probably 6hr+ in reality with all factors in), that is not a low RPE for a FM, so you'd also be in latter pens which starts later, i think with so very little buffer, you can REALLY hit that period (post 9am ish) in which the sun is really !#@!@#. Again, vicious cycle.
Not sure how well you are able to oxidise and use fats instead of glycogen, but some people really just bonk and can barely walk when their glycogen stores are used up. While some others still can jog (very slowly) and hence the cut off time might not be an issue.
For my kid, i know when he bonks hard, he needs to sit down one, walk also got issue, then i bring him fuel, first and foremost any type of soda water.

Unfortunately that also means at the moment he still would find it hard to cut body fats.
Crampings due to long extended efforts, this one i dont know how to train as i am not a cramper (never cramped before actually). Depends on your body and muscles training and race pacing (too hard in the beginning), neuromusclar even, in fact i came across one factor might be genetics predisposition even. Proper hydration/electrolytes intake may or may not matter sometimes.
In short, you would not wanna face GI issues, this is not a 1hr 1.5hrs race. You might still survive a 2hr+ HM without anything, but even though i am not an expert .......dont think FM many recreational runners can.
Anyway, not an expert at all in marathons, never run once....the longest race i have participated spanned merely just over 2hrs for a duathlon as i am not an endurace kia. But i have vomitted a few times before coz of fueling and maybe even water/drinks ingestion. Mainly is VO2max sessions, Z2 still ok. It's probably a lot based on intensity/RPE. (maybe = coz never vomit in actuality, but nearly vomit can already feel the stuff up there in my chest reading to projectile out)