I'm in the same boat with a MUCH shorter commute. The best bike route I've mapped out is about 9 miles but always includes at least 1 "dangerous" road at some point. On the other hand I can ride 6.2 miles to the train stop and then make 3 stops on the train and be right under my building.

is there another train stop further down you could ride to? Maybe a 5-10 mile ride then jump on the train? That way if you're tired on the way home, ride it all the way out, if you feel good stop early and ride more?