I Am 25 And Have Been Training For A Marathon.

I am 25 and have been training for a marathon. About 5 days ago I was running on a treadmill when I started to get a pain in my calf. Then it became a sharp pain. I stopped at 3 miles but typically i can do 7-8 miles without a much difficulty. I rested, iced, stretched for 4 days and went for a run today because I was not having any pain. I made it 2 miles and I could feel my calf tighten up and then at 4 miles the sharp pain started again so I stopped. I stretched afterwards and noticed my right calf has a dent on the medial surface at the inferior portion of the muscle belly. I can walk without a limp about an hour after exercise. I can still go up on my toes but there is a little discomfort. I am figuring a grade I calf strain but the dent concerns me a little. Any insight?