Just to echo what others have said, I think a marathon would be achievable. I normally run halfs anywhere between 2hrs-2.15
I did my first marathon this spring, no pressure about times just wanted to finish.
Its mainly a mental race, the training will get you used to being on our feet for the length of time.
One thing I would say though is that the training is a lot to commit to and it can be all-consuming the closer to the day you get.