My 69 year old butt, which has not been treated to padded shorts yet, has a range of about 20 miles, even with several stops. If I were planning multiple 20 mile runs, I would likely be trying a set. Have heard good things about them.
I have tried the Bikeroo, and a couple of the Cloud 9 seats. Currently running the largest Cloud 9 available (didn't know there were several models going into it).
I'm also running a suspension seat post, made by a company called Thudbuster, the long travel (LT) version. This DID dampen the vibration my butt was getting noticeably, making for a noticeably more comfortable ride.
From there, I would try several air pressures in the 'Runner's fat tires. The wife and I will be trying out a couple of RAD Rovers this fall - to see how we like those fat tires regarding ride, as well as sandy trails. -Al