When I first started running, only my calf muscles ever got the burn. But now, my thighs have been getting a little sore as well. Could it be because I am running faster? (I don't have a stopwatch or anything to time myself with)

See, whenever you do sprint work, all the burn is in your quads and hams; they are doing almost all the work, and you're barely even using your calves. But, when ever you jog, your calves are doing most of the work and your quads & hams aren't doing anything.