Ischemia can mean anything from normal restrictions that advance slowly with age to severe restrictions that are borderline life-threatening. People do collapse during marathons due to heart attacks associated with their ischemia. Some have even collapsed on the finishing line and found later to have severe ischemia (eg 90% blockages). For people with moderate restrictions (shown in an angiogram, for example), or that have some form of angina, or stents, exertion in the form of a marathon is almost certainly quite risky. Trying to push through the angina barrier is definitely dumb and extremely risky. Pain in the form of angina cannot always be relied on as a threshold for danger. Not advisable at all in my opinion. Slow jogging, without pressure to perform, and over shorter distances, gradually and very carefully built up over several months or longer, might be okay, but I believe still carries a risk.
As Sam has said the first stop is your physician.
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