I rode the New England Classic with a mother and daughter one year. The mother bought a bike for the ride. She did not ride but was into spinning.
We did back to back centuries and on the second day we climbed the Kank at about 85 miles. I think it is something like a 12 mile climb. I'll never forget watching the mother spin over the top at about 90 RPM not realizing that she was probably one of the only riders who did or could.
The moral is spinning builds more than I think a lot of people realize. I've never tried it at I travel a great deal so I don't belong to any groups or gyms.