Back in 1981, when I first learned to play D&D (now in retrospect, I realize it was a strange amalgamation of Holmes, original and Advanced), I was a young person in a town with few players, no "store" and a small circle of friends - thus, when I was taught D&D, it was generally one player and one DM. The player would play an entire party of adventures, stating each round what each one was doing...I actually at at the time believed that this was how the game was supposed to be played - then I went to Gen-Con in 1983. I realize that this style certainly minimizes "role-playing", but we had great fun, played through many of the wonderful early TSR campaigns. I have never heard anyone else speak about this and was curious if anyone else "mistakenly" played like this early on.
I had much the same situation as you, and when I first started out playing, I had no one to play with. Like playing yourself in chess, I created characters and ran them through published adventures. It wasn't until middle school that I acquired friends who had played or were playing.
Yup. I was from a small town. Sometimes we'd have four players, sometimes one, so we all quickly got used to playing multiple characters, even the whole party. I actually had done this more recently, as well. I ran my son, daughter, and her friend through The Lost Mines of Phandelver, from the D&D 5e boxset. We got through a good chunk of it before the friend moved away, and my son lost interest. We finished it with my daughter playing all six characters as they each were playing two to begin with.
I'm sure we did this when it was a one-on-one game, but I was lucky in that we almost always had at least three or four to play - I remember often playing two PCs when it was a small player group. Oddly enough, I'm playing this way in a play-by-post on purpose - it's a huge dungeon crawl where you play the whole party so you don't have to wait on slower players, or players that might do things differently than you. I'm having a blast with it, and I've only killed three characters so far!
If solo play is your thing, Citadel of Blood is worth looking into. It's designed so players can either play solo or everyone gets a character. The dungeon is self generating. If anyone is interested I can run a game in 2020. https://boardgamegeek.com/boardgameversion/155545/sanders-print-play-edition