I'm turning 53 this Friday and got totally hooked on SS a couple years ago. I currently run 48/17 drivetrain. Mostly flats, but some hills here and there.
I did what other guys suggested - looked at my most frequently used gear ratio on my geared bike - and approximated it for starters. After a month or so of dedicated riding, I started spinning out of gear, especially with a tail wind
.
I started getting smaller fixed cogs - to do a test ride and see if I can still climb those hills. The cogs are much cheaper to replace than a decent freewheel. (I use White Industries. Not cheap).
Good luck and enjoy your new SS!