I'll just go ahead and echo everyone else by saying get more riding time. If your longest ride is 35 miles, I wouldn't attempt a century any time soon. Maybe make your first goal a metric century (62 miles) then go from there.
The longest I've rode is a half century, once, and that took me about 4 hours (total. Riding time was about 3.5 hours), I'd like to improve that time to be able to complete it in 3 hours total before I try doubling the distance (equaling a full imperial century in 6 hours).