Modern European pilgrims on the Camino de Santiago seem to do about 20 km/day [1] and it's considered tiring but accessible by average people, not serious hikers. They carry less of their supplies and do not need to fortify their camp against barbarians.
Roman soldiers were presumably fitter and better trained than the average Roman (though maybe not by much, since everyone worked manual jobs). Maybe that's enough to account for the difference.
[0] https://www.quora.com/Did-Roman-legions-actually-march-20-mi...
[1] https://caminoways.com/how-many-days-does-it-take-to-walk-th...