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.
From your description, I'd suggest that part of the problem is that your injury isn't properly recovered yet. You might want to see a physio and/or get a sports massage, see if you can get the injury totally sorted, otherwise it's likely to recur or you'll run in an unbalanced manner to 'save' the injured area and then risk injuring something else.
I agree with marrow that you'd do best to ditch the first intended marathon. And unless you get your leg properly sorted you're not going to make your intended pace for the other one either.
I don't think I'll be joining you all next year . So far in my recovery (now more than 11 months since the accident) my longest run has been a whole 10 miles. However, at least I have reached 10 miles, with my tendon not complaining afterwards. I feel it's probably most sensible for me not to plan any major races for next year, just keep building up slowly, maybe manage something low-key like Croydon 30 in July and Ladybower 50 next September, and look to 2016 to hopefully return to ultras properly and finally get to Comrades, two years later than planned.