A very common injury for runners is pain in the Achilles tendon (known as Achilles Tendinopathy).
The Achilles tendon connects calf muscle to the heel and and is put under a lot of pressure during running which can cause irritation.
It is generally caused by increased running, doing too much too quickly, introducing speedwork or making changes to gait or footwear.
Once again many thanks to Chris Wilson at REACT physio for this useful info sheet all about the problem.
Strengthening the area with exercises like calf raises (with added weight or resistance) can really help to avoid this common issue.
Stay Safe SE Fitness!