GKD - as in the protagonist in this brilliant story I presume?
Chapeau indeed sir - I read the report with great interest having recently come back from a failed attempt at the Tahoe 200.
Although it's nowhere near as extreme as the TdG I found a non-stop 200-mile/100-hour race is a very, very different beast from a 100-miler, (Barkley excepted) - I wish there had been electric fences on the Tahoe Rim Trail
I was about to post I thought that starting training for C2C 6-7 weeks before, (and with Christmas in the middle) was a bit short - certainly to be in 'top shape'...
Having seen booktrunk's comment, this is the way I'd go - drop C2C and concentrate on Tokyo. With all the tapering / recovery you'd miss a lot of key marathon-specific sessions - in fact, you'd probably do very little marathon-specific running from New Year to the Tokyo marathon.
Ben makes a good point - once the difficult increases, (distance, terrain, elevation, altitude etc...) then so does the possibility of not finishing.
I've gone into big races, (100-milers) without the confidence that I'd finish and given myself permission to drop at point "x" - needless to say, when things get hard that idea snowballs until it's unstoppable and I dropped even though I could have continued.
Conversely, I've also (recently) gone into a race full of bravado and confidence and unexpectedly found my limit at which point I dropped, but that's a whole different story!