BlueDolphon -- welcome to the group, and wish you weren't here
You seem to have the incontinence well under control -- keep doing the kegels and exercise -- walking is as good as anything -- but jog if you must Others here will report it does get better.
You have nerve-sparing surgery -- I have non-nerve-sparing surgery. I know there is only a faint chance of my ever getting natural erections again -- if ever. But am I discouraged? -- not a bit of it.
To put some perspective on it, about 1% of my time before surgery I needed an erection. The other 99% of my time I didn't want one -- in fact some times it would have been a serious disadvantage. Similarly, about 1% of my anatomy doesn't work any more, but the other 99% still works the way it used to. That is a great deal better than one alternative -- none of it working. Your numbers may be similar.
So this is a round-about way of saying those of us in here stressing out over ED have a great deal to be thankful for.
In your case with nerve-sparing surgery, the odds are you will recover your erectile function, but it will probably be a year or two. It would have been a big surprise if your first trial with Viagra was successful. You will have to keep working at it.
As to whether your attempt was premature -- absolutely not! Three months is too long to wait. I achieved penetration soon after the catheter came out -- yes it is possible with a complete flat tire and lots of lubrication. But try anything more than that, and it came straight out again.
I was able to achieve orgasm without erection via musterbation (and a big thank-you to the inventor of that word!). Eventually one month after surgery we (my wife and I) had complete success with the aid of an injection. Since then two more successes, two partial successes and about 8 complete failures.
You may not feel like injections -- and they certainly are scary at first and I still have injection anxiety. Consider a pump (VED) -- many doctors recommend them for exercise, and I find it makes musterbation a lot easier too. And of course there is a lot of other enjoyable sensual/sexual activity that a willing couple can indulge in that doesn't involve penile penetration.
So at this early stage, keep trying -- but expect nothing and you won't be disappointed.