Have you done a marathon before? A sub 3 hour marathon is a very big ask, even for an experienced runner. If you can run a sub 85 min half marathon, theoretically, you should be capable of a sub 3hr marathon….if you’re prepared to train 5 or 6 days a week, and put in speedwork, hills, long threshold pace runs, then maybe, just maybe.
(I’ve done a 83 min half, and my best marathon time is 3:13….I think it would have been a big jump for me to get that under 3 hours).