There are those hearty runners with lungs of steel who prefer inhaling the fumes along South Dixie Highway. But the adage "What doesn't kill you makes you stronger" isn't likely to apply to the respiratory and cardiovascular systems. Besides, most of us get our daily recommended dose of automobile exhaust with no effort at all. This metropolis has precious few pathways left where the air is virtually pristine. None is purer than the place where the Atlantic Ocean, that vast watery plain where cars and trucks fear to tread, delivers its well-traveled winds. Specifically with the jogger's winged feet in mind, the sandscape that begins at 21st Street and stretches north is optimal. A large public parking lot is located just west of the boardwalk, whose planks offer a firmer firmament than the beach itself. But one can also dash through the sand without requiring a one-horse open sleigh because it tends to be packed alongside the dunes. Floodlights shining down from the mountain range of Collins Avenue condos and hotels make night jogs a pleasant alternative, especially in the heavy heat of summer.