I would submit the idea of using a dedicated ripping implement like a potato plow/middle buster to speed up the work. Use that tool to make several passes to get your width where you like it, and then use the method outlined by Ken45101. You could make the ditch however wide you want it, surroundings permitting, and keep a relatively shallow slope. Even a turning plow will make a pretty nice ditch, very quickly, when reworked with a box blade.
Around here, a wide slope like that would be instantly full of pocket gophers, taking advantage of the downslope to dump their tunnel spoils. That might be a worse danger to your horse than a smaller, sharper ditch cut with just the middle buster and/or a corner of the box blade.
Your conditions, experience, and implement situation will affect the time, but I think a realistic estimate of getting a suitable ditch done with my similar Yanmar machines would range from 45 minutes to GWDixon's estimate of 30 to 40 hours. I think trying to cut it with just a box blade really might take that long, while using a middle buster or turning plow to make a couple trips each direction, and a box blade to smooth it out, would take under an hour.
How much water does it need to carry? An honest 1 foot deep trench seems like it would be carrying a LOT of water. Is there any other way to ameliorate your ditch requirements?