Two runners collapsed and died at the Philadelphia Marathon on Sunday, eclipsing an event that drew more than 25,000. Cardiologists said that heart attack was unlikely to be the cause. More plausible, they said, was that the runners had undetected heart problems that were exacerbated by the rigor of the race. "I keep telling people, if they're a runner, they should, in fact, get a checkup and make sure they understand their risks," said Alfred Bove, a Temple cardiologist who has run marathons.