Pain in back of calf

I took up running some months ago, to raise my fitness and lose weight (I am 46, female, overweight). Recently after about 2 miles of slow running I felt a pain deep in the back of my calf – just below the main bulge of the calf muscle. I had to walk the rest of the way, and it hurt going down stairs for a few days. When it seemed better, I tried to run again but got the same pain after a few minutes of gentle warm-up running. Can anyone tell me what I have done and what the best treatment is? I am aiming to run a half-marathon next year and need to keep increasing my distance so don’t want to rest too long!
Thanks