Great job Scooty!! I just did 55 mi at 55 yrs old recently. Feels good.

Canklecat, you got it man!! If you can do 20-40 miles a couple times per week, you have 58 easily!
Pick good weather, hydrate, pick a good route, pace yourself and just do it! You will be surprised.
I'll bet on a 35 to 40 mile ride, you are on a plateau that will carry you well into the 50s.
Just pace yourself, pick the best gears, and you got it!