I think you're leaving it a bit late not starting training until near the end of November. I'd start fitting in some longer runs now, if I was you. 75K is not much less than 50 miles; if you look at http://www.trailrunevents.com/ul/schedule-50m.asp that should give you an idea of training. Of course, everyone varies and schedules are a good indication rather than rules to be followed rigidly, but if you don't get some miles on your legs then I think it's unlikely you're going to come through the 75K in a fit state to run a marathon four weeks later.
An alternative would be to train for your marathon, get in some long days out walking as well and aim for a run-walk strategy with plenty of walking for Country to Capital?
I'm with cougie and DazTheSlug on this. I prefer to choose an event which will push me, train for it properly and know that I should be able to finish it. Doesn't mean I won't DNF due to something going wrong, but I don't see that going into it knowing that I might very well not be up to it is going to help me.
I didn't know that I would be able to finish my first 50-miler, but I knew I'd trained properly and ought to be able to. And if I can get my ankle sorted enough to get back to ultras I'd like to go for a 100-miler sometime, and I won't know whether or not I'll be able to do one until I try.