Where am I right now cycling/physical fitness wise? As I said, I've been mostly off the bike for a while (20-30 mile rides at most once or twice a month over the last year/year and a half). I know I can do 2 back-to-back days of metric centuries in under 5 hours with little residual discomfort - but that is such a short distance, it's probably all but meaningless wrt 1200km training. So, I believe that I could probably knock out a 200km in 9-10 hours but would need a couple days recovery to be without any soreness.
Water purifier? Why didn't I think of that!?!?!? Great idea. Also looked at some youtube videos of "overseas" long distance brevets and noted people drinking bottled water/carbonated beverages/beer (not my preference, but it could work, I guess). So, there are at least three definite possibilities to consider that I hadn't thought of.
I could ride "official" 200-400km brevets within the local RUSA area (less than an hour's drive) but would need to drive at least 5 hours to "go further" officially brevet-wise. Realistically, there are relatively low-traffic, out-and-back routes for 100-400kms that are rideable right from my door, so I don't need official events to train for those distances.
The mostly likely venues for "official" 600km brevets for me would be either Dallas or Houston areas but I think they're scheduled too early for me ramping up from basically nothing/where I currently am. I could be wrong about that. I was kinda thinking to hit a single 600km ride about 6 months from now, an 800km ride 7 months on and then just "making the jump". Between the 800 and the 1200 rides, I was thinking of "maintenance rides" of 200 or 300... But, that's a rough rough rough "plan" (really just sticking numbers into a spreadsheet and looking at a calendar without any clue of whether or not I can make particular distance goals on that "schedule" - and including backing off distance-riding > 200km for 2-3 weeks prior to the 1200 event).
I could probably put a 600km solo training route together myself using MapMyRide or something similar without too much trouble - actually, writing that just brought a general idea for such a route to mind.
This is still mostly in the thinking out loud stage still, but now I am feeling more like it's possible than not.
Additional comments, thoughts, suggestions definitely welcome and appreciated.