My own wife has done just what you're contemplating.
Took her 4 months to get certified (2 months - medical terminology, 2 months - transcription). Then she had to beat the bushes a little to get her foot in the door with a couple of doctors (the transcription school was very helpful). She did a little pro bono work at a local (major) hospital here, partly for experience and partly to do a little "networking" with a number of doctors. The hospital required that she work on site, but as I said, the schmoozing made it worthwhile.
She now has regular work from one particular doctor, and all in all is quite happy with the situation. We have two small kids in school, and the ability to work in her dubious free time, any time, is a blessing. Remember -- ya have to buy your own equipment (transcriber unit) and some expensive reference books, but then you're all set. Our only snafu was that we bought a machine and then she wound up with a doctor who used a different format tape.
The first machine was easy to sell, however, and we made out pretty well.
Years back, wifey got her real estate license and gave that a shot for about a year. Turned out to be the year that the market died, and she sold a total of zip, nada,
nuttin'.
All this for a woman with a master's degree in molecular biology. Turns out scientists like that don't make much money. /w3tcompact/icons/eyes.gif
Go figure.