One of the hottest trends for boards right now is the mid length twin fin pin. See any video with Torren Martyn.
If you only see your self as a weekend warrior than I would still plan on the longboard. You will catch more waves and have more fun. Plenty of long board style waves here even when bigger and there are plenty of short board only style waves as well where even a mid length would be ill advised, and everything in between.
You can always sell the longboard very easily once you are ready to move on.
If you are committed to surfing a lot, then starting with a mid length is fine.
I learned to surf at 20 on a 7'6 mid length surfing every day for a summer on the Big Island in Hawaii. I felt pretty damn confident by the end of that summer. Back to Vancouver where I was only surfing maybe one weekend every month. Lost all my strength and would always take me a day to find my balance again. Bought a long board. Instantly felt way better as an infrequent surfer. More fun. More time riding waves. Once I started spending more time on the island due to my wife's work in my mid to late twenties I started surfing way more and progressed to wider shortboards. Now mid thirties surfing as much as work and family allows with a quiver of short boards for everything from thigh high mush to slabby, proper, scary, barreling waves. Everyone's progression is different and it really depends how much you can surf.
My wife for instance surfs nothing but 2 long boards here on the island. She has a high performance one for slightly bigger, steeper, but still long boardable surf and she has a nose rider for the small days, but there are obviously waves that she can't surf, but she has no interest in those types of waves.
We should make a fin thread. Insane how much fins change a board.