Hi Peter, looks like you’ve got good advice from Wetter is Better! - I’m also currently starting to build my mileage again after what sounds like a similar issue. Mine came on in the last mile of a twelve mile run.
Saw a physio who diagnosed it as Myofascial tightness brought on by overuse - a combination of tight calves, breaking in new work shoes etc. The advice I was given was no running until I could walk without any discomfort, at the same time plenty of calf stretching, self massage, heat (used a hot water bottle) and ice (rolling a frozen bottle of water under your foot). Then start back slowly.