Hello Good Evening,
Since this was your first run in around a month, it means your muscles and body were not fit for so exhaustive exertion.
In some cases sudden forceful exertion can cause muscle tear or bone fractures
In your case it seems like a calf muscle tear, due to pulling of the muscles because of such a long time exertion without a practice, but stress fracture
of the tibia
bone also can not be excluded without a x-ray of the left leg.
So go for a x-ray to rule out a bony injury
, and when it is confirmed that only muscular injury is there, then do hot fomentation of that site for the edema to subside and keep elevated the limb at most of time by keep a pillow under the leg when you are in supine position and wrap a carp bandage.
Take some painkiller medications, which also cause as anti-inflammatory, and will help in reducing the edema and pain.
Dr. Naresh Jain.