Go for it!
The last couple of entries in the “Who has Done a Hundred-Mile Ride” thread are from riders who did it as part of MS150 rides.
What makes these rides perfect, in my mind, for a century attempt is that the back of the pack is filled with riders for whom this is a giant goal, and who will struggle in at just about the same pace as a unicyclist. They’ll probably manage a slightly faster pace on the bike, but you can make that back by lingering less at the rest stops.
These rides are well supported, you’ll gain a LOT of motivation from the rock star status granted you for the day, and you’ll have SAG support if you have to throw in the towel early.
Can you get ready by August? Without question. The biggest limiting factor for most folks in saddle soreness, I think, so just putting in all the miles you can to get used to sitting in the saddle for long periods is key.