120 bpm for marching is a "customary" standard for marching bands, typically using approximately a 16 to 18 inch "step."

Music, when used for marching troops is normally played at about 60 bpm (the same music) with a step to each beat. Since musical accompaniment is "unusual" for troop movements, the pace frequently "creeps up" when there's a band, but seldom past about 80 bpm. My recollection is that the FM (Army Drill Manual) specified a 29 inch "step" for marching troops for use in calculating how fast a group would cover ground.

Sort of like the difference between music and bluegrass.

(It has been noted that several of J. P. Sousa's Marches make fairly decent Foxtrots if you double the speed. As Marches, they're usually played "faster than he wrote them," about "foxtrot" tempo when done as "musical performances" rather than as troop marches.)

John