That's great you ride to work every day!

As for training, my son and I just eased our way upwards gradually. Once we'd done a 65 mile ride, we could judge how tired we were (not terribly) and figured we could do 100 miles.

Then, once we'd done 100 miles in a day, we realized that we actually felt fine also -- so guessed we could do two of them back-to-back.

The only complication that emerged with stringing multiple 100-mile days back to back were really a) your butt gets pretty sore after day 2, so you have to use some balm and get out of your saddle a bit, and b) we both got some hand numbness -- his went away pretty quickly after the trip was over, mine lingered for two months then (mostly but not entirely) subsided. I'm probably going to change up my handlebar style for my next big ride to avoid pressure on the ulnar nerve, which is what causes finger numbness after a long ride ...

Overall, though, we just eased into it!