Originally Posted by Camel
Sounds as though you got yourself into a fun organized ride.
Ride as much as you can daily between now and then, without hurting yourself, nor hating it. Try to get at least 1 equivalent (80 mile) ride in too. Ease up, but don't completely stop riding the week before.
As far as I know, most organized rides these days require
helmets, you might want to look into that.
Have fun!
Of course, YMMV, but, for me, there is no such thing as a "fun" organized ride! Someone else picks the agenda, someone else decides the mileage, someone else decides on the route, someone else...well you get the picture. Organized rides kill any sense of adventure or spontanity involved in touring. You can't stop or take an inviting side road or change your mind in midstream. You must start "here", end "there" and not miss any of the planned stops in between. If you want to visit the Abbey at Iona... tough luck. If you want to visit the Isle of Staffa...sorry, it's off the route. If you want to spend a day fishing a nice stream...hey, we have mileage to cover. Can't let you be skooging off to do other stuff! Must stay on schedule!
As for doing the organized ride, Camel's advice is solid. Go look at the Bicycling Magazine website for advice on training for centuries. They have a good schedule for working your way up to higher mileages. Who knows, you might even have fun. It's just not my cup of tea.