You don't have to be too tough to do randonees on fixed. I did a 400km last weekend and a 300 the fortnight before, both on fixed.
American Kent Peterson
http://www.63xc.com/kentp/kentrand.htm and Brit Phil Chadwick
http://www.fixed.org.uk/ do lots of fixed brevets. There are plenty of others. Audax UK even have a couple of awards purely for fixed riders
http://www.audax.uk.net/fwc/index.htm