Astro studiously inquires:
What’s the best way for an adult to work on learning other languages on their own time?
Have a torrid affair with a very attractive member of the opposite sex who is a native speaker of that language. Maybe you’ll pick it up, maybe you won’t, but it’s pretty much a win-win situation regardless.
Granted, learning Latin this way is tough and will probably get you arrested, as the authorities take a dim view of extreme intergenerational love, especially when shovels are involved, but the true devotee of continuing intellectual development will not let such petty difficulties get in his way.