This site has a nice short little blurb about conditioning and such.
Riding 2 days a week will probably not be good to get your horse into shape. It might be enough once you have your horse in shape. But it does not seem like a fair/safe way to get your horse into shape.
So much depends on your horse's living situation and how much time you have to prepare.
I got a quite chubby, not-been-worked-for-a-year arab competition-ready only riding twice a week.. but she is pasture-kept and I took several months to prepare us both.
There is some great information about conditioning found here and many useful links here.