I agree with all those who say to stagger rides M/W/F at first. I myself am new to commuting and have a 19 mile RT ride. When I started, my knees used to get really sore (not sure if I was pushing too hard, which I was, or if my knees were just weak, which they were), but after putting in 1600 miles in the last year, I'm now able to ride 5 days in a row (which I did for the first time last week), without blowing my knees out, by making sure to not push myself too hard when I know I'll be riding several days back to back to back.