Hey, don't worry too much about it.
I think before my MS150 (MS135! We took the short route the 2nd day), I rode 2-3 times a week, but never did a ride over 40/50 miles.
My boyfriend rode only 1-2 times a week and did maybe 1, 40 mile ride. And he had never rode 2 days back to back.
We made it just fine. Sure, he was very miserable on the second day... and I wish he would have just taken the SAG wagon and met me at the next reststop... but he made it through.
The point of the ride though, is really to raise money for a good cause. It's not a race... just a ride.
The first year I did it, due to knee pain I only rode 65 miles over the 2 days. I still had fun and enjoyed the experience.
Do what you can do... and good luck!
"Life is not a journey to the grave with the intention of arriving safely in a pretty and well preserved body, but rather, to skid in broadside thoroughly used-up, totally worn out, and loudly proclaiming: WOW WHAT A RIDE!!!!"