@15 miles each way here... I ride hard in the mornings, it's too hot in the afternoon. It seems easier to ride productively in the morning, anyway--fewer cars, less need to stop, easier to roll lights and maintain speed. I find it helpful to have everything ready to go the night before, so I'm calm, focused, and ready to give a solid effort in the a.m.

I'm training for rando rides, so I can't tell you about the race-training impact. Commuting with purpose has made me a stronger rider, though. The time economy is nice, if you want to get a couple hundred miles in every week.