Can you register for the long ride, see how you feel this week/weekend and then change your mind? My instinct is to go for it, but I am not always the smartest tool in the shed when it comes to progressive training.

If your long ride has been 50 miles - less than half the distance of the ride - then ask yourself: How did you feel (really) in that 50 miler? Could you have gone another 50 - 60 miles? What was your pace? COuld you have spent all day out there on the bike if necessary? How long ago was the 50 miler?

You are trying to progress in a linear fashion toward the ride in August. It's not always linear, so you don't necessarily have to say no to the longer ride. But only you know in your gut whether you'll be ready for the century next weeekend or if it is just too much too soon.