Estimating Mileage for Grand Canyon

Going up is just a climb (think 1hr = 1000 ft for most), but going down so much faster. The Tonto is relatively flat and mostly just a bit dusty. One could make some miles if some of the water sources are flowing. Remember in one of our camps, we had to follow a stream bed to refill at the Colorado. Also our March trip was in the middle of a heat wave, so we concentrated on speed so we could get under some shade at 2 pm for siestas. Also all of us were from desert cities … Albuquerque, Las Cruces, El Paso. Those from more temperate climes were mostly worse off. Then again it can snow on the Tonto…

I know very few GC hikers (none, actually) who think of hiking in the canyon in terms of "miles per day". Maybe trail runners or corridor rim-to-rim'ers do but not veteran backpackers. The mileage is only part of the day, then there are side canyons to be explored and water to be found. I only plan 10+ miles for the day if that's how water sources work out. I'm also a fan of layovers in spots that are particularly interesting.

If you insist on the math, however, 50-60% is probably a good rule of thumb. Trails vary a lot though; I can get down South Kaibab with a full pack in 3 hours but would need double that or more for Boucher. There are trails in the GC that are rougher than most off-trail terrain elsewhere. My general rule of thumb is 1.5mhp on ascents, 2mph on descents, 3mph on flats (e.g. Tonto.) Those numbers allow for some rest stops.

In February and early March, you have snow and ice at the upper elevations to consider, usually the top two miles or so. Plan for traction devices.

IMO, if you're planning an itinerary to cover the maximum number of miles per day, you're missing half the point of the Grand Canyon and it's probably going to be the better half.

"… but how about elsewhere? I guess if we use me as an example I can comfortably hike around 30 MPD on trail. Would 60% of that be a good estimator or still too much?"

A lot depends on the route.
R2R2R – walking 2.5 mph is fine. If you haven't done fast downhill walking though, you can develop ITB (and other) issues real fast.
Bushwacking through cliff bands and prickly pear – 0.5 mph wouldn't surprise me.
Most of the Tonto – 2.5 mph is doable, but you're risking puncture wounds from cactus, and lacerations from cats claw.

The other issue, if you are off the main corridoor, is the time to find water.

More specifics will yield better answers.