I never actively try to convert, just advise people who ask. I hope that my example shows that is is easily do-able.
I would never advise a complete newbie to try a 2x6mile commute without preparation. This is bound to result in some pain and discomfort. They need to start off with short distances at the weekend, maybe 2-3 miles at first. You need to prime them for the fact that they will be slow and shaky and show (not tell) them some of the survival tricks of good roadcraft.
I am a firm believer in "normal cycling", not a an extreme sport or endurance event, just a pootle to work.