Fribley

I have been posting here for a few months now. I have a question or just want to pick some brains here. I was looking around on the internet last night for bike clubs and such and i stumbled across a list of organized rides in MI. The one that interested me was the "One Day Race Across Michigan" (ODRAM) on Sept 8th, self supported. Its 152 miles of road riding. I am very interested in trying it out, however i am a little unsure. I have only been biking since May and thus far my longest ride ever was 65 miles. I would have all of the rest of July, and all of August to prepare for it but i think that still might not be enough time for someone as green as me to the sport. I ride a Trek 7100 and i would definitely try to find a full-fledged road bike before the race (might be hard with my budget though). I am also a pretty big clyde at 5'9" 260 lbs. So i guess i am just asking does this seem doable to anyone, or should i sit it out this year and try it next year with more cycling under my belt? If it is possible what kind of training regiment would one propose? I think if i were to do it i would be pushing myself to the limit of what i would realisticly be able to do.