Like everyone says, what you're wanting to do is do-able. If you're wanting a plan, try the Ultraladies 50K, although the starting milege for the long run is rather high, or the Run for the Toad plan. I used a combination of those to go from HM to 50K - have alook and they should give you an indication of what you'll need to do.
From my experience, as you go up the miles, on each long run the "new" last mile or two is the hardest - the mile or two that were the last ones on the previous long run are already easier! Back-to-backs are hard work but I found them useful.
Don't forget to have rest days (I have two a week) and drop-down (easier) weeks.
Also, if you want to follow a schedule, start it at least two weeks early (so start a 20-week schedule 22 weeks before the race), to allow for time off with a cold or a niggle or whatever!