Isn't the rule of thumb for training for a distance to add about 10% per week? So if you can start with around 20 miles this week, then 22 miles next week, then 10% more each following week, you would be up to 75 to 80 miles by the end of July. So it does sound within reach. It's just going to require commitment.

The 20 miles to start might seem to be a lot. But by the time you're doing 35 or 40, you'll be thinking that the 20 miles was nothing!!!

I assume it is the type of ride where you solicit sponsorship, right? Well, tell your prospective sponsors that you will be attempting the 150 mile ride. Since it is a good cause, the sponsors donate even if you don't finish. So it's all good! Just do your best.

The ride is most likely fully supported (meaning that if you can't finish, they have a "sag wagon" to pick you up). There's no shame in riding back in the wagon. Just do what you can do and know that you're doing it for a great cause! I do occasional fun-rides. Some raise money for worthy causes, others raise money for profit-minded organizations. I prefer the ones where my entry fee and sponsor donations help a cause I believe in.

Good luck and let us know about your training progress.