Mimi, on any of your commuting days do you ride really, really stupidly and boringly slow? At the end of Saturday's ride, I essentially time-trialed for 25 miles. I put out a big effort. I got on the bike Sunday (last) night and my hamstrings were killing me. I rode 13 miles at a "really, really stupidly and boringly slow" pace. 8 miles into it my legs felt fabulous and I could have put in a lot more miles. Today they feel great, but if I had ridden with more intensity Sunday I would neither have built more muscle nor given my legs any kind of recovery and they would have been sore.

Sore legs every day for me would mean I'm not varying what I do enough. Sore legs every day do not build muscle. Sore legs can also be that you are not warming up properply. An 8-12 mile commute at our, ahem, age is going to mostly be warm-up. And a 7-day riding week without really focusing on "really, really stupidly and boringly slow" recovery rides is going to kill anyone's legs, especially if you don't plan a serious taper week.