30 miles is a nice distance. I can keep my attention to the riding / pedaling high, concentrating on cadence-cadence-cadence. Unfortunately, I haven't done my 30-mile cadence / TT route for several years -- and it shows in my avg pace on the longer rides.
140-kms is a nice distance. Perception is that it takes no more energy than 100-kms, and only takes an extra hour to hour-and-a-half -- not all day.
[Quoting local RUSA #1609 when he was editor of "American Randonneur" -- "regarding 200, it's not the distance, it's the time."]
Beyond 140k, the rides are fun or not depending on several factors, not necessarily in this order:
- ride partner(s), and sometimes just your shadow is the best, other times not,
- the course,
- in-shape-ness quotient.
For me, beyond 300-kms, the rides become a different kind of fun.