As the others have mentioned, you can do pretty well with a 12MP sensor at large print sizes. Landscapes are a bit tougher than other types of shots because of the fine detail, but even those can be very good. Tools like Perfect Resize will increase the likelihood of good results if you use its presets for landscapes and then follow-up with sharpening intended for finely detailed subjects (plus output sharpening). If you don't have Perfect Resize or don't want to buy it (it's not cheap), adding some mid-tone contrast can help with perceptions of sharpness. There are a bunch of tutorials on the internet regarding how to do it manually, most oriented to Photoshop, but you can adapt the techniques to whatever you're using. If your software tools include a clarity slider, that's similar to mid-tone contrast.
Keep in mind that most viewers (pretty much everyone but you), won't be quite as picky about sharpness as you. Most photographers, and that includes me, worry more about sharpness than we should.