If you've done a metric and have until September to prepare for a full century, you'll be more than fine. As someone said, do a long ride once a week where you add 5 to 10 miles each time until you do somewhere around 65 to 80 miles a couple of weeks before. You can even break those long rides in half. Like instead of doing one 70 mile ride, do two 40 back to back. This is a training technique dh uses sometimes and I have done, depending on how much time you have to ride, etc.

You should try getting at least a couple of decent rides in during the week. I shoot for no less than 20 for a "short" ride, especially when training for long distances. It's all about getting "base miles" in. The more miles you get in, the better you'll be able to do the century. If you find your schedule isn't allowing for decent length rides, do shorter more intense rides. Add hills, intervals, etc.

And speaking of hills, make sure you know what kind of climbing is involved, if any, in your century and prepare for those, too.

One of the things I had to deal with in preparing for my first century was simply getting used to be on my bike for long periods of time. Once I hit 30 or 40 miles, I got more and more uncomfortable on the bike. My neck ached and my body just screamed to get off the bike. A professional bike fit helped a lot, but getting those base miles in also helped a lot. The more I rode, the more comfortable I got on the bike. When I realized how miserable I felt on a 60 mile ride before my first planned century, I decided to switch to the metric for that event and postponed my first full century to one a month later. It was the right decision for me. I enjoyed that metric so much more and was much better prepared for the full.

When you do a century, it's all about getting to the next SAG. Don't think about doing 100 miles. Think about doing the first 20, or however many it is to the first SAG. I always look at the route sheet and know how many miles to the next SAG, and that's my goal. Before you know it, you're doing your last stretch to the finish.

Good luck! I'm sure you'll do great.