How to train for a 300km bike ride under 16 hours?? CYCLISTS ANSWER!!?

[Pander]

i am 16, currently unfit and love riding my bike. i live in melbourne in Australia, and my girlfriend lives in Wodonga, some 300km to the north east. I wish to one day this year ride my bike to Wodonga in one day, prefferably under 16 hours. At the moment i plan to ride my GIANT mountain bike, and bring a pump, spare tubes, water bottles and snacks. i plan to ride along the Hume Highway which is quite flat, but my route is undecided. i just wanted to know if i am being crazy by doing it on a mountain bike considering my time limit. how much training should i do? and what should be diet be? thank you