This (fun) site assumes 10 miles per day, while adding the necessary disclaimer "it depends".
However, I was not able to confirm this number in the cited source: John Pebbie's The Roman War Machine seems to refer to "10 miles" only in specific relation to a march undertaken by Caesar's army on its final approach to the Battle of Sabis.
The same book contains several concrete numbers (including references to primary sources) for speeds perhaps achievable by the Roman postal service and by individuals traveling on important missions. Here is an example:
Plutarch relates that Julius Caesar on one occasion travelled 100
miles a day for eight days in succession, driving in a hired raeda.
It relates that cavalry ("widely deployed in a protective screen around the army on the march, and penetrating deeper into the surrounding countryside") could have covered 40 miles a day. It also quotes Vegetius (5th century CE) on the practice of training marches with complete armor over distances of ten miles from a camp plus return:
Decem milia passuum armati instructique omnibus telis pedites militari
gradu ire ac redire iubebantur in castra ...
And FYI, here is a later account (again from Andrew Wheatcroft's The Enemy at the Gate: Habsburgs, Ottomans, and the Battle for Europe) that quantifies daily distances traveled by an Ottoman army in the 17th century:
The whole force could advance only at the pace of ox carts and the
cannon, perhaps twelve miles a day.