Hi Phil, congrats on your new hip.
I'm a former road and mtb racer, although I stopped years ago before onset of hip issues. Even though I no longer race I still ride at a high level, regularly keeping up with strong riders 20 years younger. The point is, if you did long mtb races before you should have no problem doing them again after you are fully healed. It will all depend on your level of fitness. Your hips should not limit you in my opinion. Of course a lot of this hinges on the surgeon doing his job correctly. If he messes up, well then you could have issues. Lets hope that's not the case, the overwhelming majority of people have no problems.
I'm 58 years old and ride a rigid 29er on rocky/rooty technical NE trails. My hips never hurt, although sometimes I feel it takes bit longer to get loose and warm up. That could be old age though, lol!
Hope this helps and good luck, moe