Ignorance is excusable, stupidity is another matter.

It's called a cyclometer and they run from $20 to $300 depending on the bells and whistles. Much better than the old fashioned cable speedometers of the 70's. I have one of those old monsters and is yours free for the shipping.

Check the sponsors for options. I liked the cateye astrale for a starter. You can get it with cadence which turns out to be helpful for under $30 with tax. I would suggest checkingout at LBS as it's so much a personal prefence thing. Some fonts are harder to read that others and size of font is an issue some times.

The free alternative is to get a map, or use google ped map, and measure or click out your route and see how far you went. The google map takes time, but is fairly accurate.