This is a rather common problem also known as “biomechanical overload syndrome”. Typically pain and or tightness in the calf starts gradually and worsens with distance and time and improves with rest.
This can happen for a variety of reasons:
Physiotherapy involves assessing the body as a whole. Yes we will examine your calf strength and endurance, but also look higher or lower to investigate what else may be contributing such as weakness in your thighs and buttocks, which may be placing more stress on the calf complex. In addition there may be weakness in the muscles of the toes or some tightness in the nerves supplying the area.
Symptoms can improve with activity modification, exercises to improve the capacity of your calf to absorb loads and perhaps changes to your running style but this can take time so as ever be patient!