That happened to me the day I rode 10 hours straight through...on the road but on on rough chip seal.

What do I do different now? Chamois butter, better shorts, lift off the saddle a bit more often for the rough spots and change position every couple of minutes - even just to shift weight a little.

I havnt been on the bike for 10 hours straight again yet, but I already have my plan set for when I do!

I feel your pain!

ETA: Oh - ice for the first 48 hours, then alternating ice and heat - whatever feels better. I keep saying I am going to market an ice pack in the shape of a bike saddle!