Hey David,
What Brian said in his initial post really is the kicker with this backpack for your situation I feel.
But will 2,400 cubes be enough for your gear?
To put this into perspective: The most common backpack on the PCT and AT for many many years has been the ULA Circuit and its larger brother the Catalyst. They are exceptionally comfortable backpacks and therefore significantly heavier then the Arc Blast, ZPacks heaviest backpack.
The ULA Circuit is 4,200 cubic inches and the ULA Catalyst is 4,600 cubic inches.
Lets take the ZPacks Arc Blast at 3,200 cubic inches, and add on two hip pockets to obtain another 122 cubic inches, then just for the sake of it lets add onto the backpack two top side pockets and you can gain another 100 cubic inches or so of backpack space. Put all of this together and you are at around 3422 cubic inches of space. That is almost 780 cubic inches of space less then the smaller ULA Circuit. That equates out to two or three days worth of food depending on your PPPPD ratio. I think you could also think of that as about the same size as the SMD Skyscape Trekker.
I have owned a little over a dozen backpacks from ZPacks and as far as comfort goes the Arc Blast supports a heavier load better then my other ones. For me a heavy load is anything over ten pounds.
I have yet to put any miles on my Arc Blast with a TPW of 25+ pounds.
The reason I have not is because I cannot fathom how to get 25 pounds worth of gear inside of this backpack.
I recently loaded it up for a five day hiking trip and my TPW was in the 15 pound range. I had the backpack *stuffed* and I ended up clipping on the outside a couple of stuff sacks with other gear inside of them. This was for a sub-freezing weather hike so I had a 20 degree bag, a puffy jacket, and a large air inflated pad inside. I had just enough room for five days worth of food.
If you would like to see just how much gear you can get into one of these Arc Blast backpacks I did up a video on my setup that you are welcome to watch, and compare your gear to the what I have inside (to compare bulk sizes) and hopefully from there you can determine if all of your gear will fit in and on this backpack.
If I were to buy another one I would have them add three inches of material to the top so I could (a) gain another two days worth of food, (b) have additional winter gear on the inside rather than hanging it on the outside, (c) not have to compress my sleeping bag and puffy jacket so much. I would also add the top side pockets to gain another 100 or so cubic inches of space. And of course have the hipbelt pockets. Adding these to this backpack and I think this could be an very viable long distance backpack (which is all I usually care about, not sure what your uses will be, but I am mostly a long distance backwoods hiker).
Lastly: I have had a number of people (experienced hikers and non-hikers) put this backpack on (loaded with the 15 pound setup in my video) and every comment has been how comfortable the backpack holds those 15 pounds. For me with thousands of miles, I find myself thinking I am missing some gear, as it feels more like it has 7 or 8 pounds inside of it, rather than 15. The way it distributes the load has highly impressed me. But as I said above, how it will handle its rated 30 pounds TPW is beyond me, as I simply have no way of getting 30 pounds of gear inside of this ~3450 cubic inch backpack.
–Abela