Personally, I wouldn’t put much into a marathon prediction from a 5k. Now if you had a recent half marathon that was predicting a 3:10 full marathon, that would be a different story and very realistic. If you have time, suggest racing a half before your full marathon to gauge where your longer distance fitness really is.
I raced a half in May & managed 1:32:57, that's what gave me the idea for thinking i was fit enough for the 3:09-3:10 range my last one, but the heat conditions and the throwing up just after 10k made me adjust my pace drastically & turned into just trying to finish & not have to DNF or faint. I have my 1st 10K next week & a 10 miler the following weekend. Hoping to get Sub-70Min for the 10 miler, & as close to Sun-40 as i can for the 10K, but think I'm a little off that type of result atm from barely making the Sub-20. I do think a sub-70 is pretty doable from the Half result in May & how that type of pace has felt in training since, but I'm actually kind of nervous for the 10K😂, which might sound ridiculous but i'll go out at Sub-40 pace & try hang on for as long as possible