There are too many answers to that question. The mountain bike came out in the late 70's and early 80's. Subsequently, narrow-tired drop-barred bikes became less common. Before this happened, pretty much every bike was a road bike, and the majority of those had what we now call sport-touring bikes with 40-inch wheelbases, for frames of about 23" seat tubes. Bikes that seasoned long distance tourists preferred tended to be a bit longer, but I've done plenty of touring on sport-touring bikes and done fine. I've even done long distance touring on road racing frames with wheelbases of about 39". I had to modify my stroke to prevent my heels from hitting the panniers, and steering downhill at high speed with a full load took a lot of work, but it wasn't impossible as people predicted. It just wasn't ideal.

So if you find any frame from the 80's, it is likely to have that in-between geometry that we call sport-touring, especially if it's not a high-end frame.

So this thread has inspired me. If I can gather the spokes and rim cheaply, I'm going to build up a wheel with an AW hub and put it on my fixed gear bike. I'll need to find a cable and shifter somewhere, too. I suppose I'll end up putting upright handlebars on it. Right now, it has drop bars. For the full effect, I should put on fenders and maybe even a chainguard.

It's a cheap, generic bike I got for a mere $300 from Nashbar. There are no decals on it at all. It was so cheap, I had to replace the pedals and tires immediately.