I did 60 miles once (30/30). The thing I didn't like is that after sitting on my butt for 8 hours at work, I got really stiff. The ride home was kinda miserable. Make sure you stretch/cooldown well on the first leg.

(Now I just typically go one way (palo alto -> pleasanton) and get a ride back. the folder is great for this. I'll typically leave early and get close to 50 miles if I'm feeling frisky)