Ars Magica 5th edition repeatedly uses the "pace" as a unit of measurement. It is never directly defined anywhere in the book. Indirectly, from one or two references, one can deduce that at least some of the authors probably intended it as three "feet" (probably assuming a foot to be the modern US foot, and the pace the modern US yard). But note that neither the "foot" nor the "mile" are ever defined anywhere in the book.
I think there's a better way to define paces, feet and miles, given the whole emphasis about the roman roots of the Order - an order nominally dedicated to the roman patron god of travel, no less! And that's to use the (very precise) roman definitions of pace, mile and foot. It takes some very very minor errata to fix this in the corebook: removing two parenthetical remarks (that directly or indirectly equate X paces to 3X feet) where nobody would go looking for the info anyway, and inserting these notions into the glossary on p.8.
A pace (latin "passus") was the standardized distance between [equivalent points, such as the tip of the toes, in] two consecutive footprints of the same foot by a roman legionary marching at full stride, very slighly less than 1.5m. Note that it's much cleverer to measure this "double step", rather than a single step between footprints of different feet, because if you connect same-foot consecutive footprints of a person marching straight you get a straight line, while if you connect alternating-foot consecutive footprints you generally get a zig-zag. It's the former, not the latter, that must match throughout a column of marching soldiers to keep it together.
The pace was a crucial unit for measuring travel distances; we know exactly how long it was because the Romans peppered their roads with army-friendly markers reporting distances to major landmarks in thousands of paces. Today we'd say a "Kilopace", the romans said "mille (1000) passus (paces)". That's what the root of the word mile is - "a thousand [paces]". A roman mile (1000 paces) was thus very slightly less than 1.5Km, i.e. somewhat shorter than the modern US mile.
A pace was further subdivided into 5 subunits (the Romans where fond of 2s, 5s and 10s), and since each was roughly the size of a footprint, one fifth of a pace was named a "pes" (plur. "pedes"), the "foot". The roman foot (0.2 paces) was thus very slightly less than 0.3m, i.e. slightly less than the modern US foot. Note that while the english words "pace" and "mile" come from the latin words "passus" and "mille (passus)", the english foot is independent of the equivalent roman word of "pes", though for obvious reasons they measure lengths that are pretty close.