I'm baffled

Before my surgery, Dr. B said my heart was doing an 8 minute mile while I was at rest. How is it that with a properly functioning circulatory system, I couldn't run an 8 minute mile now if my life depended on it? I run (and I use the term loosely) closer to an 8 minute half-mile! Is it just me or does this seem horribly, horribly wrong. (Which were probably the exact words going through the minds of every car that passed me tonight . . . horribly, horribly wrong.)