I haven't read all the responses, but ...

1) Make sure your bicycle fits you

2) Start building up your distances. Those of us who do centuries and randonnees didn't do 6 miles a day 4 to 5 days a week, and then all of a sudden went out and rode a century ... we may have started with 6 miles a day, 4-5 days a week, but then we started adding 10% per week to our distances, and gradually built up to longer distances. I predict that the day you cover 12 miles all in one go, your 6 mile commute will really start to seem easier.