Running and taking Warfarin

I will take Warfarin for the rest of my life, I have been told. All the questions for DVT/PE do not cover a DVT just happening. My question is this; I like to run, but find that I feel as if I am not getting enough oxygen when I do. I get upper chest pain and am lightheaded for a few hours after I run. Does taking Warfarin effect the amount of oxygen carrying capacity in the blood? The lower leg swelling that existed for about 3 months before the event and the swelling after the event has finally returned to a near normal. Event Feb 2004, DVT/PE lost approx. 20 minutes of my life, felt as if I were hit with a bowling ball in the chest, extreme difficulty breathing, Xrays showed a peppering of PE in both lungs. Result Warfarin for the rest of my life. I feel much better when i run but as of late, am having difficulty running the same distances as before. I have run many races up to 1/2 marathons since later part of 2004, but now the training runs seem more difficult, not harder. My intake is checked monthly by the VA, and I take 5 mg X 5 days and 7.5 mg X 2 days... normal levels are 2.0 - 3.0... average has been 2.5 or above. Greg R.