This thread is becoming weird. There is no evidence that ancient armies or people of those times could march any longer than modern, fit soldiers can. As already hinted at, any distance requirement in ancient sources concerning marching ability cannot be held as a requirement for sustained marching unless it is specifically reported as such. Modern armies have those kinds of high-end requirements as well but only for single efforts in times of great haste.
I do not know the correct term in English for it, but in my officer training we talked about marching casualties. That means men that either become unable to advance any further or become too exhausted or sore to perform an effective fighting duty at the end of the march. It is a serious matter. If there is no transportation available, the entire unit either has to stop altogether or just abandon their men at the side of the road. The latter will destroy any morale of the troops, because everyone will be thinking whether they are the next one to be abandoned.
Harold's march south 27 miles a day is based on a calculation of an ancient account that we do not know much about. We do not know if there were marching casualties and how they were dealt with, and we do not know if the account is reliable. What we do know is that they were beaten at the destination. And there is an understanding that the stationary defenders, not having marched, were deliberately making retreats to further wear out the already worn-out attacker after a prolonged forced march.
I am not sure what you mean by slaves occasionally, but the common perception of Romans employing galley slaves is an early modern fiction from the 17th and 18th centuries. A warship was an extremely expensive and strategically important resource, and its power and steering capability was not supplied by a force of starving and abused slaves with no motivation. In Roman navies, trained marine soldiers took turns to row with set rest periods. It was customary for military officials and officers catching a ride from port to port to participate in rowing together with their personal servants that could be of slave status but not the abused kind. Those were the main instances of slaves participating in rowing Roman navy ships. In addition to that, there were times of shortage of manpower when slaves were employed with the promise of freedom for their services in the navy, but that does not conform to the modern fiction of forced and expendable slave labor manning the oars.