I too would like to know the underlying logic behind his suggestion. One thing I have noticed about some cardiologists is that they simply do not understand athletes, esp endurance athletes. Endurance athletes are a particularly perverse breed because they constantly subject themselves to pronlonged exposure to pain. My current cardiologist is pretty good at understanding my desire to return to my former level of fitness, but he too can add in his own subjective valuations into our converstations. For example, when I was finally given permission to start training 100% again (my surgeon told me to go ahead a week earlier), he suggested I not take up Muay Thai (Thai Kickboxing) as I wanted to - not because my sternum was not strong enough, nor that it posed any special threat to my heart, but because Muay Thai was a particularily brutal art where one can easily break bones, etc ... My responce was that I knew this; this was the reason I decided to get into Muay Thai. Doctors, who seem to be in the position of fixing our problems, sometimes fail to understand that a broken rib or a little knee pain are things we would gladly endure because we get something greater out of the activity as a whole. </rant>
Talk to your cardio, find out why he imposed this limitation! Perhaps there is something he is looking out for. Im not a runner by nature, but in the last few weeks I have been running to keep in shape (and to strengthen/condition my shins further for Muay Thai) as I wait for my new bike to arrive, and the prospect of a marathon has entered into my head once or twice (a mile!). I will certaintly get back into long ridse. Centuries, double centuries! Im not stopping because of something like avr and I dont think anyone should give up something they love without understand why first!