Adjusting the Training for 50 milers & 12 hr duo

I guess I wasn't thinking it through too much when I told Donna YUP - I'm in for 12 hours duo. After a couple days of letting it sink it, I realized I shoulda been in those 3-hr spin sessions twice a week with the Mountainside gang all winter long. Last year my endurance was perfect for the long stuff after 4 months of 10-12hr weeks on the bike only; but this year is a different story having been doing only 1 3-hr session maybe every other week, getting of total of about 5-7 hrs biking 'cause I'm also running and swimming 3+ hrs a week. So it may be tricky building the bike back up to assure I can make it through 12 hours as a duo. But I'll certainly give it whirl - there are 11 weeks left to prepare. AND, the other side of the coin is once I have that endurance and long-ride readiness in my back pocket, why let it go to waste?! I should do a 50-miler or two...so I might end up doing RB Winter (LUV that park), instead of the 1st Xterra of the season...just not sure yet (chrissy - don't kill me!). Of course I could try both too (RB Winter is Saturday and the Xterra is Sunday). Ouch - that would really, really hurt. And I may do the Stoopid 50 too if I feel up to it - they have an old gals category! Time will tell on the training. Guess I better get off the computer and start working out!