Just did GC R3 on Saturday, and ran today for the first time this week. My feet are still a bit blistered, but legs are okay. So, if I had to do an Iron build starting now it'd be no problem. The main thing is GC is so long you don't run very fast, so I've felt worse after marathons.

This was my first ultra distance run, and fitness was derived primarily from an April marathon build. 18 hours is definitely enough. Our only plan was to get out early (4:30am), constantly fill up on the water, hit the sodium tabs on a consistent basis, and continue fueling.

The only thing that I'd do differently is go with a lodge over a tent (at the end of the day you'll want to use the shower and a softer mattress), but in that case you have to plan much further ahead.

IM and GC are both life-altering experiences, and I highly recommend them both!

Depends on how hard you do R2R2R for the recovery time. It is a hell of a training day and will only help your fitness. With your fitness 18 hours might be a good guess, but it might take you longer 20-21 hours depending on how deep you want to dig. Its not a graded event... just do it for the experience.

Training plan? JFR and JFR hills. And do some hiking, not all of the trail is runable.

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Suffer Well.

I did R2R2R last May, about 3 weeks after the St. George 70.3. I had an OK run base going, but was not nearly trained up for a 50 miler. We started at 10:15 pm from the North Rim and ran up the Bright Angel Trail because it had water available. Came back the same route. I was around 16.5 hours total. I've done two IM's (IMSG '11, '12) and I was more wiped out after the Canyon run than either IM. I am doing Vineman in July and I didn't join my friends who ran the Canyon last weekend. I didn't think it would be good fit for me right now. You could be totally fine though if you've got a strong base.
I will say that running the Canyon was truly a life changing event. It is one of those things I look back on frequently and I still get a smile on my face.

It's fun, that's for sure. But I don't think it's too specific for Ironman Training. You could potentially practice your nutrition. The recovery, for me, wasn't too long, but boy the hike was awesome!!!!!

Like others have said, depends how hard you push it. If you are just hiking and doing a little running here and there, you should be good to go right away. If you are really pushing the pace, it might take a week or two to recover. (add additional time for blisters!!)
When I did R2R2R, the crew I was with all ran Oil Creek 100 the next weekend without any issues.