Dilemma in SCSM08

I am interested to join the 12hr walk event in Putrajaya on 13-Dec. If I join, this will be my first endurance competitive walk event. Judging from my Sundown pace, I should have good result. However, the StanChart Singapore Marathon is on 7-Dec, just one week before. I also want to join the SCSM. However, if I join a 42km, I am likely not full recovered for the 12hr event. Should I take a step back to 21km or even just 10km as a practice?