I think there's a danger of overthinking the specifics. At the end of the day, the key challenge is still to have enough endurance to be able to run (and/or walk) a bloody long way, so I reckon a good proportion of your training can be exactly the same as for a "normal" marathon, but liberally sprinkled with some terrain-specific sessions.
When I trained for the Beachy Head Marathon (3,500ft ascent I think, so not quite as harcore!) I made roughly half my long runs off-road and hilly. The longest run was 23 miles with the last 6 miles (before warm-down) being effectively a hill-repeat session stuck on the end. Then I substituted a hilly fartlek in place of my usual MP/tempo run mid-week, maybe every other week. Generally 10-12 miles total, running easy on the flat, hard up each hill and recovery jog down, repeat...
ES - Actually, looking at the Garmin stats afterwards I now think it was also due to not having warmed up properly. (I rode my bike to track rather than the usual 3 mile jog, got there bit later than planned so only a lap warm-up before MP.) For both miles, cadence was identical at 185, so it obviously felt right but it took a while for my stride to get going. I find cadence correlates very strongly with pace, with easy pace being in the 170s, MP mid 180s, 5k about 188.
I'm unaffiliated but it is possible to get your results listed on Runbritain/ Po10. I only count PBs from those that are listed on the above sites.
Don't forget, there is a distinction between UKA licence certification and course measurement certification. If the race has a course measurement certificate, I'd certainly take this for official PB purposes regardless of whether it's listed in Po10. But it is nice having everything in one place.
Po10 is definitely more inclusive than it used to be, even before considering the 50 squillion parkrun results it includes. I used to use the minimum requirements as targets; SG I think the 5k time was 16:40? (The slowest 5,000m time I have listed for 2009 is 16:39, and I definitely ran slower that year.) Certainly between Po10 and the RunBritain site, they've got most things covered.
From a race organiser's point of view, I believe ARC has (or had) its advantages over UKA in terms of less paperwork for H&S, etc. and details of insurance cover, but let's not go down that political hot potato of worm-cannery, especially as I don't really know what I'm talking about.