ahh.. don't you just love the smell of the AT in the springtime...
while i actually like to trek (shorter ones in my case) both in the dark and in the rain, unfortunately, spring on many parts of the AT has a downside.
just remember two words applicable to some parts of the AT in the spring:
rain [a lot of it] -- issue: down vs. synthetics, or how to keep down dry, or perhaps more accurately (since it will probably get wet), how to dry it out - if it's raining non-stop mod-hvy, a fire won't dry out your gear even if you can build one large enough. since, in some cases drying just ain't gonna' happen for a while, how to keep it as dry as possible is a worthwhile consideration, viz. sleeping bag with Epic fabric, DWR, UL WPB bivy are choices, etc.; also rain gear, tarp size considerations, also ground sheet and digging rain channels vs. bathtub floored tarptents. read DrJ's Lost Coast experience - perhaps worse than most days/nights on the AT in the spring, but some can be prit' near as bad or equally wet. oh, and on rare occasions i've seen fog into the mid morning hours, so thick, and on one occasion again in the afternoon as the heavens let loose on the already saturated earth with trails under a 1/8" stream of moving water, that i couldn't see the lowest branches of trees above me, nor further than 10'-12' (3m-4m) in front of me.
frankly, when you're surrounded by such high humidity for days on end, [nearly] everything in your pack (even inside of a pack liner) ends up, at best, damp, if not wet - compression/release cycles of your pack as you move, not to mention opening it, unpacking, packing, and closing it each day, causes air to be pumped out and humid air in which is now in contact with gear that might be somewhat hygroscopic. perhaps one of those ULA packs B.F. made for the "Terrific Trio" on their somewhat recent Alaskan Trek might be in order, but then it's not cuben and weighs a bit more, but it's something to consider. can you make cuben dry-sacks, Bill?
mud [a lot of it, and sometimes, i do mean a lot - had a trail runner sucked off my foot in ankle deep mud] -- issue: wet feet, spare socks, & sufficient foot care products. sometimes, you have to go off-trail, into growing green, dripping wet foliage to avoid the mud on the trail. this slows you down and gets your lower legs and feet soaked, plus makes a poncho/PT difficult to use.
be prepared accordingly. sometimes, literally, DAYS straight of rain w/little let up - i've seen up to five days straight of mostly mod-heavy rain; nothing dries out and it can still get a tad nippy a night. so, can have lots of rain which produces lots of mud which produces a poor and slippery trail surface, especially on rocks, AND slower going or a bad fall or sprain can end the endeavor - keep that in mind when planning resupply and time of year to thru-hike the AT.
oh, and a third word as the temps warm a bit... [drum roll please]
bugs [did "bit" give this one it away? it was an intended clue], issue: bug netting and/or bug dope mandatory.
lastly (this one i have NOT personally experienced, but have talked to enough people who apparently have), Mt. Katahdin to Springer Mtn in the spring: in upper New England, particularly Maine, think SNOW MELT and SWOLLEN RIVERS (sometimes chest deep) that need to be forded - mud can be shin to knee deep in places as those waters recede; all equates to slow going. Depending upon when you start and if the temps are warmer than normal will determine when and how much of this snow melt occurs - so, you'll want to plan to leave Katahdin to avoid it (either early or late). Plan on a crossing of the 100mile wilderness to take longer than you might normally expect and plan accordingly.
there's a reason most people select the proper time of year to go S-to-N on an AT thru-hike. this is NOT to say that you can't do it as you plan to. Bill, you being an ex-Army lifer (Airborne/Ranger???), i'm sure that you are up to the challenge.
also, keep in mind a second rainy season starts in the AUTUMN/FALL out here in the northeast. while not as rainy as the spring, we are currently going through five days straight of rain, some times mod-hvy 0.5"-1.0" per hour at its peak and 1"-2", or more, per day of rainfall. this again makes many of the trails quite muddy at times.