aka it's not a good idea to run through pain.
More experienced runners should know their own bodies and when to investigate something that feels wrong. If you are in pain, it is a good idea to wonder why you are in pain so that you can decide if you need to rest or perhaps to get some medical advice. If you diagnose the problem early, you may be able to get away with simply resting whereas a pain that has been made worse by more running can stop people running for weeks or longer.
For many new runners to Ealing Half Marathon, the problem could be simply doing too much running in a short period of time and you may be able to solve it by resting and starting again later.