Longest to date is 140 miles but I'm not one for stopping that much so could probably manage more.
The real issue though, like average speed, is not some universal figure that applies everywhere to every situation. You say 60 miles and I can think of plenty of places where 60 miles would be more tiring than 120 elsewhere - the Pyrenees, the Alps, Exmoor, Dartmoor or in contrast somewhere very flat..? Where are you riding? What is the objective?
If the objective is to keep on riding further and further then you'll probably surprise yourself before too long as you obviously have a degree of base fitness already. But some go for endurance events/sportives where distance itself is of the essence and others prefer higher speeds over a shorter distance, so it all depends on what you want to achieve...