Well, it sounds like what you are doing is good for the weekdays. Are you able to work out with weights? Upper body strength is an important component in longer rides.

You have some time to train for the century in August. Unless there is a lot of climbing involved, the only thing to really be concerned about is time on your bike. Figure that it will probably take at least 6-7hours riding time to complete the ride. So, you will need to do longer and longer rides before that time. This will allow you to find out how your body reacts to being in the saddle for long periods of time, as well as what kind of food and drink works for you.

Yes, a club is a good idea. I suggest you look at www.bikejournal.com, a website which has folks from all over the world. (You can join for free,although premier membership helps the administrator keep the site going. Our group from here is Team Estrogen.)

Anyway, after you have joined you could start a thread asking about the century you want to do as well as people in your area that ride. They are a great group, I have met many people(including some of the gals from here) and I have high praise for each and every person. I'm sure a few riders will join in with advice and routes as well as clubs they would recommend that are near where you live.

Good luck on your endeavors! It's a big step, but it's also a huge accomplishment!