There are some training plans out there for doing your first century (100 miles). You could follow one of those (maybe scale it back a bit), but those plans tend to just follow the adage that you should increase your rides by about 10 percent each week.
I, too, followed the couch potato to 5k a few years back, but cycling doesn't lend itself to quite that structured of a plan. In my opinion, there are a few variables--terrain, weather and wind--that are more noticable on a bike and generally have to accounted for. In other words, mileage doesn't tell the whole story. It's also easier on a bike to make bigger and bigger leaps as your fitness improves because cycling just isn't as hard on your body as running (thank God). Thus, it's not quite as necessary as a beginner to follow a real structured plan.
So, I would suggest just devising your own general plan. If you're doing 8 miles at a time this week, try 10 next week, maybe 12 the following and then 15. So on and so on. As you do longer and longer rides, you'll have to start integrating more food and drink.
Good luck!