I have a similar distance commute that I ride several days per week and here is my advice. Since you are only planning on commuting one day per week my recommendation would be to use the rides as recovery day. The problem (for me at least) with a ride of that distance is that it goes by so quickly its hard to get in a decent work out, when I try I always end up neglecting something, i.e. no time to warm up, no time to cool down or the if I leave time for both of those the workout time gets too short. If I don't get a good warm up my high intensity efforts go poorly and if I don't cool down properly then my legs feel bad for the rest of the day.

So what I do is just use the morning as an easy ride at a easy pace and extend the ride home to make it a more usefull time and distance. Just my 2 cents.

Hope this helps. Don B