Whipp, Ward and Hassall 1998 wrote an interesting piece on the average Roman legionnaire. Keep in mind that most of our ancestors lived much more physical lives than we do now, estimating their work capacities becomes very interesting and can demonstrate how far we can go.
After some brief translation from Roman Imperial units into modern units, we see that the average legionnaire was 5 ft 9 (this was in an era when the common man was 5ft 4) and weighed ~180lb. Two marching paces are detailed, full step-, which is with no load, and military step, which involves 60lb of gear (armour, weapons, food). Experiments on university-aged men of similar size resulted in a mean VO2 1.43l/min for both rates. How the ancient Romans determined how quickly to walk with and without armour to work at the same rate can only be imagined. Based on the data collected, it was determined that no legionnaire had a VO2max of less than 40ml/kg/min- since falling behind was punishable by death at that time. If the worst soldier in the entire unit had a VO2max in the low 40s, the unit as a whole would have performed excellently at endurance tasks even with load over distance. This would explain why, when matched against the physically more imposing Germanic and Gaulish tribes, the Legions were able to quickly outperform and outmanoeuvre them.