Hard breathing plus cold, dry air irritates the nasal lining, and the lining answers with mucus.
The NHS lists exercising and weather changes as common causes of non-allergic rhinitis, the runny or blocked nose that has nothing to do with allergy. It is normal physiology, not poor fitness. Breathing through the nose on easy efforts and wearing a buff in cold weather take the edge off. A constantly runny or blocked nose outside exercise, one-sided blockage, or frequent nosebleeds is worth a pharmacist or GP conversation, since non-allergic rhinitis usually responds to sprays.
What to do
Common questions
Why do I get a runny nose when I run?
Your nose is doing its job, just loudly. When you run, you breathe far more air per minute, often through your mouth and nose together, and in cool or dry conditions that air irritates the nasal lining, which responds by producing extra mucus. The NHS lists exercising and changes in weather, including temperature and humidity, among the common causes of non-allergic rhinitis, which is exactly the runny, sometimes blocked nose you get without any allergy involved. It is so common it has an informal name, runner's nose, and it is not a sign of poor fitness. Simple things reduce it: breathe through your nose as much as pace allows on easy runs, wear a light buff over your nose and mouth in cold weather to warm and moisten the air, and run at lower-intensity times of day when pollen or pollution is heavy if those make it worse. If your nose runs constantly, not just during exercise, or you also get sneezing fits and itchy eyes around pollen, dust, or animals, that is allergic rhinitis territory, and a pharmacist can suggest sprays or antihistamines. See a GP if one side is persistently blocked, you get regular nosebleeds, or it is affecting sleep.