A century ride is actually pretty doable. It's somewhere between 5-10 hours for most individual, possibly a tad less if you're good and are riding with a group. You mostly just have to keep moving.
A double-century is a bit more challenging, but this is also a commonly encountered distance for recreational riders. Times tend more to 9-14 hours. At this distance, chafing and other discomfort can be an issue.
Elite cyclists may attempt 300 or more miles in a day, which does start to get a bit stiff.