Ironman to Ultra

i only have 1 ultra under my belt but as this has gone unanswered for a few hours, heres my .02.  are you planning on taking much time off after the IM?  most ultra training plans take you from a relatively short 'long run' and build up over several months.  you will probably have to tailor something starting at a higher weekly volume, thats more in line w/ the 8-12 weeks between your events.  i couldnt bring myself to do b2b long runs, but most ultra plans will call for it in some fashion even if the 2nd day is a reduced mileage day.  if it works for you, go for it.  if not, just do 1 long run a week.  im coming from running into tri, but if you have a fueling plan that works, i would certainly try it first before changing up your feeding strategy.  i got to the point where i could eat almost anything during a run and it didnt bother me, but i prefered whole foods to gels/endurance drinks.  yes, you WILL get hungry, my 50k trail race was like a running smorgasboard.  i think i spent almost as much time chowing food as i did running .  hopes this helps some.

i am also very interested in the responses. Looking to do a 50K 5 months after IM Lake Placid

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I am no expert but you should not have a problem. I worked a 50 miler in during my training for IMWI last year. A good milage base of both running and cycling will carry over well for 50k. You have all summer to increase your run miles and 51k is not that much futher than what you will run during IM.

I do not do back to back long runs but that has more do to my work schedule. I have done three >20mi runs in the last 2 weeks with shorter recovery runs in between. I know many ultra runners that do back to backs with good results.

As far as fuel goes for 50k. Gels and shot blocks for me with whatever sports drink is on the course. Any father than that I end up going with whatever I can get down and tastes good.
I think that an IM is about as hard on my body as a 50 miler so 51k will be a cake walk for you.

Good luck

51K is only a little longer than a marathon, so the training would be similar and there's no need to do back-to-back 20 milers or anything like that. 

What you do need to do is keep your long run at 15-20 miles and do some of them on terrain similar to what you expect for the run.  Pace for most ultras is substantially slower than a marathon because most of the time, it's off-road.  That 20-miler on the trail may take an hour or more longer than a 20-miler on pavement. 

As for nutrition, the trail runs I've done are more an all-day snack.  You can probably get through 51K without solid foods, but you may enjoy the run more if you slow down a little and take in some solid food.  Practice a few times mid-way through your training run (start very conservatively, one food and really slow down and try other foods and faster paces when you've had a success or two). 

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If you're training for a 50-miler, then some back-to-back long runs (30 miles split between the two days) would be good. 

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Recovery is a personal issue and related to your other goals.  In my case, I was training for IMCdA 10 weeks after my 50-miler (I used the ultra as a long training day...it was a chance to figure out how to keep moving after 10+ hours of keeping moving. )  As such, I had a 3-hour ride the day before the ultra (not advised, but I knew it wasn't happening after) and spun for an hour on the trainer the day after the ultra. 

One of the 100-milers earlier this month was doing her 3rd consecutive 100-mile run. 

For me in 50 training I would go long on Saturday build up to a 30 mile run. On Sunday I would do a comfortable 10-13 mile run. Both of the runs would be on trails. Then I would take Monday off, do workouts Tuesday - Thursday to include one trail run, some hill repeats, and 1 or 2 double run days, then Friday off. I would build up for 3 weeks then take a 40-50% cut in mileage for a recovery week.

I really liked this plan, I just came up with based on what I felt.