(I surfed almost every day for 9 years and considered myself above-average in skill.)
As others have said, it's a lot like riding powder, and turning a snowboard is similar to turning a surfboard, except that you don't have to initiate the turn with your front foot, you can just lean, thanks to the skegs. It's probably more complicated than that, but that is one big difference I noticed between the two.
But the thing that's going to screw you over the worst initially is weak paddling. Whenever I go back to surfing after a hiatus, this is the thing that makes it the most difficult. Being able to paddle hard for a long time is crucial because, without it, you cannot get yourself into the right takeoff spot for the wave, and you cannot get the right amount of speed to prevent yourself from getting sucked up into the lip before you try to stand up.
With these two problems taken care of, learning to stand up and ride will be much easier. For this reason it may be wise to go out very often initially, even when the waves are bad, just for the sake of strengthening your paddling. Swimming should help, but I am not sure how much.
People also often recommend starting with a very big board, and this does make it easier initially, but if you plan on riding a shortboard eventually, I would just go ahead and start on a shortboard. It will be more difficult, but I think it will be best in the long run. Might as well get that adjustment over with sooner.
TL;DR: rubber arms will make it all seem a lot worse at first; get past that, and things will get much better for you!
P.S./Edit: A bigger board is easier to paddle, so the "rubber arm" problem will be less acute initially, but even so, I would recommend pushing through on a shortboard. I took a 2 month surf trip a few years ago after not having surfed for years. I don't think it took me much longer than a week (maybe two) of surfing every day to get my paddling to an acceptable level again. That said, I am not saying it's WRONG to start on a longer board; it's just my opinion. I have never taught anybody how to surf.
Last edited by frankzinger; 04-27-2014 at 04:57 AM.