^Wrong. And indoor numbers do not translate so easily to the out of doors.

This can absolutely be done on the road. You have the right idea. Just go out and do it. In two months or so, go out and do it again. In between, follow a structured plan which will be very effective in bumping up FTP, see Friel or the Racing Forum. To be sure, a 20 min test will only give you feedback about a narrow power bandwidth. If you really want to learn about your riding, do shorter tests too and longer as well. 60 min, 10 min, 5 min, 1 min, 30 s, 15 s, 5 s.

Have fun.