Fortran is simple enough to be used by dumb physicists. At its core it is a standard procedural language, like C with a lot less footguns and much more user-friendly. Its object-oriented parts have some strange syntax, but are otherwise unsurprising. There isn’t any step like when starting something like Lisp or Haskell.

It would definitely not take years to get on top of it for anyone who knows a bit of programming.