Thank you for your question. Breathing difficulties during exercise can be dependant on several factors including:
1) Your age
2) Your fitness level
3) Your general health status. If you have a medical condition that may compromise the delivery of blood and oxygen to your vital organs, you can feel breathless on exertion or during exercise. There can be many reasons for these including conditions from the lungs (e.g Asthma, bronchitis) and heart (e.g. congenital defects) to conditions of the blood (e.g. low blood level).
4) Intensity of your exercise regimen
Blocked nose from a sensitive nose (Rhinitis) should not result in shortness of breath or difficulty in breathing. You may feel "more difficult to breathe" because you have to resort to mouth breathing when your nose is blocked which may be uncomfortable. However, this shouldn't lead to inadequate oxygen flow to your lungs as your mouth and throat which is part of your upper airway should be able to cope with the increased airflow to your lungs during exercise.
We do know that uncontrolled Allergic Rhinitis can worsen Asthma and their co-existence is often known as United Airway Disease. Inflammatory mucous from the nose and sinuses (from Rhinitis or Sinusitis) can result in exacerbation of common lung conditions such as Asthma and Bronchitis. Since you mentioned that there is increased mucous in your nose, you may want to see an ENT Specialist to have your nose checked. This will likely involve a nasoendoscopy, a scope through the nose under local anaesthesia. If your ENT Specialist suspect that you may have United Airway Disease, he/she may refer you to a Lung Specialist (Respiratory Physician) who will likely perform a lung function test to assess the ability of your lungs to breathe. Certain types of Asthma can be worse during exercise.
If you have other symptoms such as chest pain, palpitation or light headedness during your run, you may have to see a Cardiologist who can perform stress tests to determine if your difficulty in breathing is due to a heart condition (less likely in a young patient like yourself unless there is an unknown congenital problem).
If after reading this and you are still unsure, perhaps a visit to a General Practitioner will be a good start.
Hope this helps and all the best!
