Old, rusty clunkers and blue jeans
At the start line for the 2008 STP, there was at least one guy on a uni-cycle and 3 folk with old clunker road bikes (complete with market basket and read baskets). They seemed to be awfully casual about the ride (they did say it was their first time) in their blue jeans and sneakers.
No, they weren't planning on a one day ride, but, as the expression goes, "better them then me"!
Yes, a person could ride STP in one day with most anything, but it is oh so much easier to !!FAIL!! by having less than decent equipment. I studiously try to avoid things like sore butt and pain in the legs, etc.
I view that decent bicycle equipment is mandatory for a one day ride, but even more important are the "details": seat, shorts, fit, etc.