Furthest you've ridden fixed is...

I've done a few spin classes. They can be a novel change from riding outside.
All the 'real' cyclists will laugh (roadies, I'm talking 'bout you) but that's because they are afraid to stray outside their little realm of hardcore.
There's plenty of totty viewage at spin class too. Mmm Mmm. :)

My SS MTB was 32:15 for the road (now THAT makes you spin!) and 32:17 for dirt 'n' hills.

I run similar on my mountain bike 36:16. To keep up with any one I have to spin as fast as I can for about 30seconds to a minute, let me rest and then do it all again once the bike slows down.