Well now that is a big claim to make.  Since my crystal ball is a little hazy lately I can't predict the future.  I'm not sure how anyone else can either.  I guess it depends on your definition of long distance.  Although the OP talks about 100+ mile treks, I would define anything over say 20 miles as fucking long distance.  To say that grunts in the future won't cover these kinds of distances is a tall prediction.

And yeah, grunts have been patrolling on foot lately.   Not only southwest asia, but southeast asia, and yeah, northeast asia as well.  Lot of that going around.  Kinda depends if you're talking light infantry, Ranger, SF, or anyone that might have to cover terrain that is not vehicle friendly.  But I would submit that big army is still married to their trucks, and tracks, and tanks.  Even the Corps has been leaning on their tracks as of late.

The key would be what gen warfare you get into.  If it's straight-up gen III stuff, like north korea barreling over the boarder again, then yeah mobility warfare rides again.  But if you have another insurgency with the G's hidden in mountains and jungles, well, prolly more apt to see more dismounted stuff.

Only time will tell.

Years ago I did a 12 mile forced march with only 2 liters of water, 40lb pack. Never again. Felt like I was gonna die after. Some of the other soldiers were failing out from dehydration from carrying too little water and not stopping to drink.

I did a 16 miler a few weeks ago with a 25 lb pack in just over 3 1/2 hours. I was well hydrated before and I drank roughly 1 liter every few miles. Felt fine afterwards and the next day.

Granted, these are forced marches. In a SHTF scenario I would be taking my time and resting some, taking better care of myself, and my feet, when METT-TC permits.

As far as how much water to carry and where to store it? 6 liters should last a few days if you pace yourself. Always have some type of micro water filter. Had a buddy in my unit in Afghanistan the last time I was there get pinned down in the mountains during an ambush. He was stuck there for hours and ended up drinking from creek water and got a parasite. He survived the ambush but almost died from the parasite. They had to send him back to Germany. Ever since then I always carry one of those little straw filters on my kit somewhere easily accessible.

I would carry a small amount, 1-3 liters, on your carrier and then the rest in some type of sustainment pack. Its just to much to put on the carrier.

I think a good plan for dismounted survival is plan for 3 days. Beyond that you will need to find more resources.

Long distance dump the Armor. Chest rig and ruck is what you're looking for.

The worst I've done is 35 lbs 44 miles in 17 hours in 90 degrees heat. Up and down the Hungarian mt/hills with really high humidity.
That broke everyone off. Started out with 3L camel back and 3 liter bottles. There was resupply along the way too.

Then 50 miles in 3 days was fun too but that had a lot of other lanes and getting smoked going on. Also had resupply points.

When deployed we worked out of Matv's, maxpro's or hmmwv's. When we patrolled and carried everything we didn't go anywhere near 100 miles.

So if I was doing what you plan on doing I would carry a light carbine style rifle, chest rig 4 mags max. a ruck that doesn't exeed 45 lbs.

Start out doing 12 miles, 20 miles, 30 miles etc. See what works for you. What gear is worth keeping and what is just dead weight.

Plan routes with someone at resupply points. Then once you get that down plan routes that have natural resupply (water that you can purify).

If I thought I absolutely had to have Armor or more ammo I would be working out of a ATV or Truck.

Edit. Once you get those 12, 20, 30 milers down. It will give you a better idea of what you need personally for water and snacks.

^^^This.  So, I do long-distance backpacking.  Most trips are in/around the Southeast (and in the summer heat/humidity).  We've done some trips out West back home (PNW) and have a few 100+mile trips planned out West as well.  Most of our backpacking trips range from 50-150 miles (mostly depends on work schedules), but the longest, no-supplied trip we did was 80 miles.  Water was the only resource we had access to which allowed us to limit our on-board capacity to 2-3 liters and frequent filtered resupplies along the trail.  Food-wise, 100 miles is feasible if you can maintain 10-15 miles a day and try to keep the time under 10 days.  The challenge comes back to water as most of the lighter food choices all require water for reconstitution and dehydration from simple exertion is significant.

As much as a I love my "battle belt", that, along with my PC would get left behind for a 100-mile trek through the back-country.  You can patrol with a pack without a waist-belt, but you're not doing long-distance without one and even if you could "suck it up" and suffer through it, I guarantee you wouldn't be worth a shit at the end of the day let-along the end of 100 miles (which few would make with that setup and a heavy load).  There are a lot of ultra-lightweight backpackers that go with minimalist packs (no real frame or waist belt), but anything over 15-20 pounds without a waist-belt, frame and suspension for proper load-bearing just isn't smart.

Water weight sucks, but is essential.  If water sources are limited to non-existent along a route I know was my primary (or secondary) travel route, I would strongly recommend water caches.  It's cheap and easy.  Even along the Appalachian Trail, some of the springs are seasonal and dry up.  We've run into situations where water wasn't available without a significant detour, which is why planning and current intel of resources is extremely important.  For my backpacking kit, I carry a few extra soft-water bottles and an extra Sawyer "squeeze" bag simply to increase water storage for those times we know we have limited access to water on some trail stretches.

So, forget the PC for distance trips.  Yes, I've done my share of road-marches with a plate carrier and ruck.  Usually the standard 12-miler (and it sucked); but you're not doing a 12-miler for 8 or 9 consecutive days with that kind of setup and load.

Personally, I us a ULA Circuit for trips I know we can get frequent resupply and there are plenty of water sources.  That allows me to keep a total pack weight below 30 pounds (with 2-3 liters of water and food for 4-5 days/40-60 miles).  For serious trips, I've upgraded to a Seek Outside Unaweep 4800.  I bought it for some trips out West where bear canisters are mandatory, but the key for the these systems are their weight capacities (not just volume capacities), which will easily support over 100 pounds (not that I want to carry that much!).  For longer sections or areas with limited water, I can actually tank-up and haul more water (hopefully shorter distances only) without stressing the pack or straining me with a pack not designed for that kind of weight....there is a difference.

For a "combat load", while I always take my HPG Kit Bag on distance backpacking trips, I would go back to my TT MAV chest harness for more serious social engagements and where I was including a long-gun.  Another option is the RIBZ chest-pack.  All integrate with a pack's waist-belt/suspension without interfering with the load bearing performance.  For light loads and shorter distances/duration, you can get by with the sustainment/combat belt and combat pack using only the shoulder harness, but for distance, it just doesn't work and I've seen first hand the results of young guys (either Active Duty or recently ETS) using their ALICE on up through more recent MOLLE packs on the trail and the results aren't pretty.  You can rely on anecdotal or third-party information, but until you've actually done it, it's simply academic and opinion.

For distance with any amount of weight, you need a properly fitted pack, a frame and suspension that is rated to carry your expected load-weights, and yes, a waist-belt which his designed to carry weight on your hips and not on your shoulders.  For a combat/fighting load, the best options I've found are from a minimalist HPG Kit Bag to any number of quality chest rigs (tactical or civilian such as the RIBZ) that will keep your loads off your waist and onto your chest without interfering with a packs waist-belt and suspension system.  These systems do put more weight on your shoulders and upper body, so it's imperative to understand what are truly the essentials for your fighting load and stick to those limits so you don't over burden your shoulders.

Heavy loads over long distance inject new enemies...dehydration, heat exhaustion, loss of calories, potential mechanical injuries, hypothermia, etc.  You need to invest far more time into planning and physical conditioning than into gear purchasing.  The right gear for the mission simply comes with experience.  Get out and start doing some backpacking to find out what actually works and what doesn't.

ROCK6

Former IDF guy...we did long distance rucks in the desert all the time (~70 or more km in a day, most at night). In that situation, armor was considered optional and most guys would slap soft armor on their rucks rather than wear it. I can't think of a single guy who wore hard armor in that situation. The prevailing logic was, it's better to show up to a fight capable than either not showing up at all, or showing up totally smoked because of the extra weight and heat. The extra weight was partially accounted for by water.