I think the best thing you can do is go along with someone who has experience.
They can challenge you to go way beyond what you would have thought possible. You need someone you can trust not to overdo it.
If you find someone like that, you can go to 30 miles and beyond in style.
Have you got any aerobic base so far? Jogging, walking, hiking, anything?
edit -I see you have been around here for a while and have biking experience. What's your current, comfortable distance?
There's a bike club in your town, they do rides, maybe call them ?
Washington Women Outdoors
19450 Caravan Drive
Germantown, MD 20874
(301) 864-3070
Their website appears to be down.