Why set an arbitary mileage target, just endeavour to use the bike whenever it is appropriate, I cant imagine car drivers set targets like that !
I do 14 miles a day, occasionally extend but generally though I enjoy it, its transport, the quickest route between two points with a byproduct of exercise. I wouldnt just do loads of extra mileage for the sake of it on the commute, sometimes your annual mileage is just down to how far you have needed to travel !
Anyway, its like when I sit uon the train, filling crosswords and Suduko in very quickly like a genius, nobody ever checks it