If you're already doing 150 miles/week, you really shouldn't have any problems getting ready for a century in September. I don't even think you'll have to ride that much more total per week to do it.
First, don't worry about how fast you go.
Second, start doing longer rides - probably on weekends depending on your schedule. Pick the weekend two weeks before the century - try to work up to about a 70-80 mile ride that weekend, of similar profile/climbing as the century will be. Once a week should be more than sufficient. Do the first one at a distance you KNOW you can do easily. With the century about three months off, start with a 30 mile ride and up it 5 miles per week. That should get you up to 70-80 miles with a few weeks to spare in case you want or need some time off the bike.
IMO the biggest thing about a century is getting your body used to putting out a continuous light to moderate effort for HOURS, especially for a rider already doing 150 miles/week.
The long distance forum? Those guys are crazy - in a good way. A mere century to them is like a walk to the mailbox for normal folks.