A ultralight 700c with very short cranks is worth considering for long distance. The low weight and small amplitude leg movement helps with fatigue.
I could ride my 700c/65mm all day 
Seat comfort is a big factor for ultra long distance. Use the shortest cranks possible…it increases resistance to give lift from the seat, and less bouncing from legs going up and down.
Not willing to hijack this thread, but this applies to long distance riding too.
Two things that come to mind: you should try to wear padded cycling short (I do it under my shorts), and tilt your saddle up at the front a little bit more. In the beginning, it’s good to have it pretty horizontal so that your weight is aligned with the seat post. But as you begin to get a good feel for riding, having it pointing up a bit is good as you really want to seat on your seat-bones (aptly named) and not at the intersection of your legs, which can indeed cause pain and numbness after a while. Curved saddles make it worse as you tend to slip in then and get stuck on your mid parts.
Alas the seat is already at maximum up tilt. And the shorts I have tried so far merely push me further forward than where I would be sitting without them. I tried climbing on with them indoors again before yesterday’s two-laps-of-the-park ride, and decided I’d be more comfortable without than with.
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Currently I’m wondering if it is sensible for unicycle saddles to be so wide at the back - if they were narrower, the bike shorts might work. Much thinner shorts would be required for me to get my weight back on the current saddle.
In terms of the thread, I think the point is that finding a setup that works may be as time consuming - and is definitely as important - as the physical training aspect.
You can always put a couple of washers at the front, between the seat post and the saddle, to make it tilt a bit more.
They’re wide at the back because that’s where you’re sitting. And as strange as it may, a flatter saddle is more confortable for distance as you don’t slide into the middle part. The Nimbus is pretty curved, especially when you compare it to a Fusion KH (and I’m not even talking about the completely flat Fusion Zero).
You’re entirely right, long distance (and even just general comfort) is a combination of training, and the right equipment.
It’s because that part is so wide that I can’t sit on it - and I end up even more forward with the bike shorts, which effectively make it yet wider.
In effect, to be able to sit on it, it would have to be not quite so wide, or there would have to be something like a handlebar providing rearward force to sit more “against” it than on it.
Yes, this is a direction I’m interested in looking into.
Another realization is that while the other halfway decent saddle I got to try (an impact naomi) is generally less comfortable than the nimbus gel, it is less curved, and I haven’t given that one a try with the bike shorts yet.
Big thanks from a newby!
A big thanks to everyone for the awesome advice and support. I took up unicycle just 2 weeks ago after a ten year ultra running career has recently been cut short by osteo-arthritis. I have 2 months to go from novice to competent, ready for this 24 hour marathon. It’s awesome to discover such a supportive and encouraging community on this forum.
As to the wheel size: as has pretty much been said:
36 if maximum speed is your goal, 29 with short cranks (<130mm) for decent tradeoff in travel size and speed. Also, with your time frame learning the 36 would be REALLY ambitious, as learning the 36 is challenging even after 1-2 years of unicycling experience! So 29 sounds like the ticket.
Unless you’ve got an open-ended budget, then a 26" schlumpf (2 gear unicycle) would probably be the most ideal: small so easy to fly with plus effective 39" wheel for speed in high gear and “bailout” low gear 26". But that’s way too expensive to buy for almost all beginners… and learning to ride high gear in 2 months would also be really really ambitious (I successfully mounted and rode a 29 schlumpf after about 6 months of riding my original 24" on and off so doable too).
As to the ambition and learning: if you come from a major ultra running background, then I think the same training, discipline, and overtraining ideas can be applied very well, e.g. getting your saddle right is like getting your shoes just right plus the right technique, which is equally or more important. overtraining is much less of an issue than in running except for the saddle area (but there are no joints or shin splints, just pain, so much easier to manage). And discipline and a training plan will be essential, but that should be easy for you.
I would learn to ride on the 20" you got. Then start with 150s on a 29 with one ride a week ramping up to more on the 29 while simultaneously working down to the shortest cranks you can handle for the event: ideally something like a 110/127 dual pedal, so you can ride fast on the 110s and if you get totally tired and can’t hack it, then you switch the pedals to 127 and it’ll seem so easy (though no change for your seat).
Provided you have the time to practice and train, I would consider this goal doable for me back before I learned to uniycle in my early 30s (i had a short running/adventure racing background plus lots and lots of cycling). Let’s see, November is 2-3 months, so I would probably need to be unicyling at least 4 times a week for 30-60 minutes plus a long 2-4 hour ride per week (i.e. like running the training for technique and then the long run), probably with some cross training of running or cycling to train your cardio before you are proficient enough on the uni (I guess cycling with your knees). You should already have sufficient cardio, so things to train are: technique ( simpy how to ride, plus how to ride efficiently and comfortable, plus muscles (legs, stomach and BACK muscles you will never know you had!), plus tolerance buildup for saddle area (technique helps but in the end you simply need time in the saddle).
Good luck!
Cyclocross tires are some of the best options for meeting in the middle in my opinion, as a cyclist. New to the unicycle still, but I have them on a couple bikes, as it means light dirt trails or road are done equally well.