No, am a Genteel Gentlemen of Leisure (?Unemployed?Self-employed with no work?Empty nesting house husband? = Let's call it retired...)
68, starting from "only" doing 20 miles, aiming for 68 miles in less than a year is a pretty stretching target.
6 hours riding is much more than 3 times 2 hours riding!
From experience, don't aim to do longer distances every ride. Stick at 30 miles for a few weeks, aiming to feel more comfortable.