I take rest days when I feel that way. For example, I had beautiful weather all week so far and commuted everyday until today, I decided to drive in because my legs feel wobbly and tired and my knees are feeling stressed. I've been commuting for years now and find 30 miles a day is alot for me to do every single day. My commute is 15 miles one way now, up from my usual 11 so rest days are needed. Best of luck and don't get rid of the car yet as they come in handy sometimes.