My CAD experience started on Computervision equipment (a big dedicated mainframe system) back in the mid-80's at a Navy shipyard. We were doing 3-D system process system piping back then. You inserted stuff with a wireframe appearance, then ran the "generate detail" command, and came back and hour or so later to see the 3-D appearance of what you'd done. Pretty cool stuff but achingly slow. I read later that whole system ran on about 4MB of ram. We'd have 6 or 7 terminals running off of that mainframe; no wonder it was slow. It had these tremendous stacked hard disk packs that you could swap out. Cool stuff at the time. Info here:
Because the Computervision system only had so many terminals and we had people waiting in line to use it, we got Autocad (R10 I believe) installed on a couple of 286's (much like the original poster's equipment), and I gradually picked Autocad up from using those. I remember discovering the first zip programs so that you could save a drawing that was larger than 360k onto multiple 5-1/4 floppys. Holy crap what a pain.
Around the early 90's the Navy decided to make a wholesale switch to Intergraph (also a mainframe system), but I was only involved briefly with that.
After a few years I moved to a new job where we flirted briefly with Microstation mainframe equipment (which was apparently what Intergraph became). I took about 3 training courses before management decided Autocad was the way to go so I moved right back to that and have been using it ever since. Oddly enough my present employer still uses a lot of PC-based Microstation.