I think I'd probably make it a better version of shadow walk. Same 50 mi/hr but minus the random return location (b/c no shadowy weirdness). I think I do like the 4xHD limit, even if it is a little harder to calculate. Duration is as long as the caprine can play, then you end up back in the Material Plane. I guess the "wandering away from the caster" penalty is a bit different, too.
Bear in mind that while
shadow walk is a 5th-level Bard spell (or 6th-level for Sor/Wiz) and allows 1 creature/CL to travel for 1 hour/CL,
etherealness is 9th-level and affects the caster plus one creature per three CLs for 1 minute/CL. Even
ethereal jaunt is 7th-level and only allows the caster to travel for a round per CL.
That suggests moving ethereally is a far more difficult magic than traversing the Plane of Shadows.
I think
March to War ought to be more limited than your suggestion to maintain some vague parity with the spells.
There's no mention in the resonance descreption of the marchers being able to move up or down through thin air or pass through solid objects, so maybe they don't
completely enter the ethereal plane and and
incorporeal rather than
ethereal, which makes also them
invisible to Prime Material creatures?
I'm OK for creatures to could travel faster than their normal hourly movement rate while under
March to War, but 50 mph is too fast for my tastes.
Let's see, an average PC can hustle at 4 or 6 miles an hour, depending on whether they have a 20 ft. or 30 ft. base speed, or half that speed if they're walking normally. That means
shadow walk is 10 or 20 times faster than a typical humanoid can travel, plus there's no risk of them tiring.
Somewhere around 10 or 20 miles an hour is more to my tastes.
As previously mentioned, I wouldn't mind making it based on the characters actual speed rather than it being a set speed à la
shadow walk. How about having them move five times their walking pace (i.e. 15 miles per hour for a speed 30 ft. creature).
Oh, and I'd only have it multiply the recipient's
land speed. It's
March to War, not
Fly to War!