I had a similar experience last year, I did a 600 mile tour in 6 days. Up til then, my longest ride was 140 miles, my second longest ride was 62 miles, and I rode two or three days a week and had been riding for a little over a year in a more serious manner. The trip was very hard, but I made it through. If speed isn't an issue, I would bet you could make the trip tomorrow, it might not be the easiest thing, but you would make it through. Just ride as much as you can, I think you will find that your legs and conditioning won't be the biggest pain, it will be your neck, shoulders and arms. Be sure to get a good bike fit and those problems will lessen some, but being on a bike for four or five hours wears on you after a couple days. Good luck, it will be something you will remember forever.