Scouts can take unwalled cities on marathon, so long as you have several and they lvl up to get the third survivalism promo. They can hit the city, lose about 5-8 health, then get 3 back the same turn (2 from survivalism and one from an adjacent medic unit). So long as you attack from a hilll/forest, you're usually OK. Or, do it with the Mongols and a Khan - scouts that heal 5 per turn is pretty cool.
However, marathon has twice as many turns as epic... I can't see it being a viable strategy. But they do still help along with warriors - promoted scouts are great at taking a share of the dmg.