THIS is the slide rule for doing time/speed/distance calculations:


Old school way: used to use a watch and count cadence over a minute. With a small chart - memorized or taped to the handlebars, you could convert cadence in a gear to speed. I never taped anything, but did have a couple of gears pretty well memorized at one point. In the midwest it's even easier, as many of the roads are at 1 mile intervals. 6 min = 10mph, 5 min = 12mph, 4 min = 15mph, 3 min = 20mph, and much above that I'm working too hard to calculate.
For distance, for me until around 1985 it was always the map.

As an aside, the first computers I recall seeing were advertised in B*c*cl*ng magazine around 1982. They were about the size of a small router and did speed and distance, and were about a hundred and seventy clams. Cateye came out around 1983/4 - neither these nor the Cateye Solar (with cadence) turned out to be particularly good in rain. I never knew of one to go more than a thousand miles or so on a tour without wiping itself out. It must have happened somewhere, just not wherever we were.

The computer brings up an interesting question. If I ride from my town to the next town and back, it's a neat 20 miles.
If I ride 15 circles in my driveway and two loops of the parking lot in the high school at the turn around point, I've still connected the exact same two points but now it's 20.5 miles. Only it isn't 20.5 miles - and I know that deep down. I don't use one now. Actually, every once in a while I'll put one on and check the distance along some route or other, but I'm happier without it. YMMV, of course.