warfarin and marathon running

QUESTION: Does warfarin affect the efficiency of your blood - I mean its capacity to carry oxgygen?

I am a 73-year-old marathon runner trying to return to fitness but my capacity to sustain extreme effort seems to have become permanently diminished.

The warfarin is for atrial fibrillation - to reduce the risk of a stroke caused by clotting due to erratic blood flow.

My resting pulse at the moment, in bed after a night's sleep, is 54 but it is NEVER steady. It is always a little bit atrial.

When  train hard, I seem to come up against an obstacle that stops me from getting extremely out of breath.  At first I thought it was lack of willpower, or fear of possible consequences.

Now I suspect it is purely biological. My body won't let me do it. I used to be able to run a mile in under six mins; now the best I can do is about 7.5 mins.  Also I can no longer get into the state where you are sobbing for breath but recover quite quickly and immediately do another hard repetition.

Do I blame warfarin? Do I blame A-fib? Do I blame old age? Any theories please.

My aim is to do my next marathon in 3-and-a-half hours, an hour faster than my current best efforts. Up to the age of 60 I could do a marathon in just three hours.