As as an allergy sufferers and a person with a nasty history of sinus infections I would also mention that it is a good idea to do whatever is necessary to make sure you can breath clearly through your nose as well as your mouth. I now just use Flonase if I really need it but I do take Zyrtec (Cetirizine) daily which I believe helps. The way I look at it, breathing through your nose and mouth optimizes incoming air (along with the belly breathing already mentioned) in a way that will allow you to make the “burning” in your legs more of a moving target, such that you can validate gains in your fitness level over time.