Pelotonia - Century Charity Ride help

Being my first year of trying triathlons and also road biking I decided I wanted to do the local charity ride which has varying distances (25miles, 50miles, 100miles, 180miles).  It is called Pelotonia (Pelotonia.org) here in Ohio and the big distance for everyone is the 100mile century ride.

So far I my longest ride ever has been 30miles.  It was not torture by any means and I felt like I could go futher if I had to or planned more time for riding that day.

My question comes about training for such a long ride which is August 20th.  I am also doing tri's (sprint and olympic) which can help in the training.  But are there any plans out there or other websites that could help in ramping up to long distances?  I know you should slowly ramp up running to not injury yourself but does the same go for distance biking?

I recently bought an indoor trainer so I can ride even when the weather is crap and also since it is a charity ride it is not a competition so I am not looking to "win" something.

My logs are up to date and I know I need to do more biking but I get my bike back from the shop today as I had some derailer issues.  Also had a trip the previous weekend which bit into my usual weekend riding.  Any tips would be appreciated!!

Joe