In addition to commuting, I generally ride a long 50-60+ mile ride most weekends. After I started commuting, my typical weekday ride was about 10 miles on each leg of my commute, compared to about 20 miles when I used to ride after work from home. In other words, I was riding 10 miles twice daily compared to 20 miles once daily. That change had no adverse effects on my performance on weekend rides. I was initially concerned that the shorter but more frequent commuting rides would lower my fitness, but it seemed to make me stronger. Like I said, commuting daily is tough work. You are carrying more weight, stopping and starting more often, and riding in more adverse weather conditions. Commuting makes you tougher.