Ditto on the whole "lowly" idea. I guarantee you that even for the ultra riders, at one point a 30 mile ride was their longest ride ever. I know I had that point in my riding career.

Build up gradually. What can you do now? 5 miles? 10 miles? If it's 10, add 3 - 5 miles on your next ride. Keep building gradually. You can get there. Keep it FUN. Stay hydrated, and remember to take in some food as you're riding longer and longer distances.