After 3 years of missing my goal at 24 hour races, I signed up for spin class on something of a lark. My experience was much the same as yours; I'm sure none of them could have ridden with me on a century, but their out of the saddle efforts were humiliating. Killer. The impact on my aerobic capability over time was obvious, and did have a significant impact on the road.
When Father's day weekend came along, I clocked my goal of 400 miles at the National 24 Hour Challenge. I think the spin class put me over the top.
That said, I'm not a fan of the music, the trainer screaming over the music, and spending a nice evening indoors.