I commute 5 days a week and do at a longer training ride on the weekend. The trick for me as others have said is to mix up the intensity. My round trip is 20mi. My typical routine is:

Monday: Normal day, steady Z3 effort
Tuesday: Hill climbing, target 40-45mi total, 3500+ feet
Wednesday: Z2 recovery for both rides
Thursday: Hill climbing like Tuesday OR HIIT session in the morning
Friday: Normal day, steady Z3 effort
Saturday: Full rest day, stay off the bike
Sunday: 40-100mi training ride with between 3,000 and 8,000' of climbing depending on the route and goal for the week.

Before I was commuting 4 days a week and taking Wednesday and Saturday off completely. I actually feel better with the Z2 rides on Wednesday.