I don’t think your legs will have anything to do with it. How are your feet going to feel? In some ways, I’m the wrong person to askI don’t like low-cut footwear for any trail use, and in most cases probably wear a boot that’s a step heavier and taller than most. It’s a personal preference, though. For you, I’d say that if your feet feel OK after ten miles, then chances are they’ll be OK after the full 24. My only comment is that if you’re thinking of the Lowa Tempest ($75; www.lowaboots.com) as some sort of shoe “upgrade,” currently you must really be wearing lightweight shoes. That’s no knock on the Tempest, but it’s a very middle-of-the-road, low-cut light hiker. I’d suggest you instead go with a high-end trail runner such as the Montrail TRS Comp ($95; www.montrail.com) or the New Balance 904 ($99; www.newbalance.com). Both are very nice shoes with lots of cushioning and stability. Just the thing for an endeavor such as the Big Ditch.
Of course, I’m sure you know what you’re getting into, so I’m reluctant to offer much more advice. But I’ll still offer a little. One, watch your fluids and don’t drink too much water. That may seem counterintuitive, but one of the biggest health problems people encounter in the Grand Canyon is called hyponatremia. In layman’s terms, this happens when people drink so much water they literally thin out their blood chemistry, resulting in too-low levels of sodium. So drink plenty water, but also eat solids and drink something like sports drinks, which contain a little sodium.
Two, brace yourself for all kinds of crap from park rangers. They hate rim-to-rim hikes, and if you encounter one on the trail who divines what you’re up to, he or she will actively discourage you from doing so. To some degree their concern is justifiedthe heat and altitude of the canyon is something many people can’t cope with, even very fit people. But it’s also a sort of creeping paternalism that infests many of the parks. So be prepared.
Lastly, have fun! I’d love to do the rim-to-rim hike some day.