ED is not guaranteed, nor is incontinence. While I did not have radiation, I had RARP with most, not all, of my nerves spared. I had an erection within hours of surgery and have maintained frequent and strong erections since.

We have very little control over the outcome but so long as the nerves remain then you have a fighting chance. You can help improve that chance with hammering your pelvic floor.

First, a story I wrote about my journey. Not very prescriptive, but might be a bit inspirational:
http://bikrutz.org/triblog/?p=3516
Next, a link to the first program I followed, which opened several doors to the process I followed to success: https://melissahadleybarrett.com/penile-rehabilitation-program/

And, the elements (without the evidence-based justification behind them) of my rehab program. Not saying it's the only, or even the best way to proceed, only that it's what I did, and I succeeded:

1. I bought and followed the program Melissa Bradley Barrett offers in the link above.
2. I bought a vacuum erection device - https://vacurect.com - and used it ~ 5 times a week, specifically, pumping up for three minutes, releasing and milking out the engorged blood from my penis for 2 minutes, pumping again two more times. Total of 15 minutes.
3. I took the prescribed sildenafil (generic Viagra) 50-100 mg about 4-5 x a week.
4. Bought a vibrator designed for a flaccid penis and used it to induce orgasm 3-4 times a week. (https://www.hotoctopuss.com/products/male-sex-toys-award-winning-and-best-selling-pulse-solo-essential-dragon-eye-vibrating-penis-masturbator-special-edition)
[#2-4 intended to keep the nerves involved in orgasm and blood flow in the penis functional while the erection nerves recovered. There is some scientific justification for each of them.]
5. During the whole process, my wife was a patient and supportive partner. While playing around in bed after the first two months was frustrating, it kept us connected as a team trying to return to whatever level of intimacy we could create at the time. Without that, I'm sure success would not have occurred.
6. I realised I was rebuilding my sexual function from scratch, and worked on each of the elements separately before trying to put them all together: read the last half of my story above for more insight.

I think it helped that my post-op PSA was "undetectable" 6 weeks, 3 and 6 months post-op. This freed up emotional bandwidth for the penile rehab as I wasn't worrying about my cancer...