Originally Posted by Old and Slow
Go for the 50 mile ride. Even if you were riding 20-30 miles a day, the 75 mile ride with 3000 feet of climbing would be very difficult.
Okay - but look who is giving this advice "
Old and Slow" 
You need advice from
"Young at Heart and Fast" 
So, here goes:
Make the 50 your goal. That is doable given your training and experience.
Save the 3,000 foot climb for next year or at least until later this summer.
Have fun!