I would try doing a nice long run on the weekends to mix up your current routine, and then on you normal runs during the week, try making it harder by making it a "fartlek" run. funny word, but it's a real running term. basically, during your run you jsut pick up your speed for a short distance (say, the lenght between 2 telephone poles) and then go back to your normal pace. You normal pace will feel hard, but soon your breathing/hearrate will return to normal. you can really fine tune the workout to your own needs. short, really fast intervals or long, medium fast, or you can go by time. Say every 5 minutes you are goign to pick it up for a minute. You will really feel a difference the next day and it can be really fun to push yourself in this way. I like to pretend I'm finishing a race while I'm doing the faster segments, and imagine people cheering me on!