One thing that has always amazed me about Bike Forums is the number of people who do most or even all of their riding alone.
I've been riding road bikes since the early 80s and most of it has been with others. Sure, after work rides are alone but I am a member of a road club and there is always someone to ride with on weekends. If not the club I have other friends who ride, too.

To answer the op I have done solo rides around 50 miles, 4 hour climbing rides alone, and multi-hour off road rides exploring new areas.