Yeah, 31 seems pretty fast for a newbie on a suspension mountain bike.
I can do upper 20's for a short while on a downhill with my Schwinn Hardrock Sport, but that gets pretty shaky and it's not worth the risk. I almost nailed a deer at 25-30 one day that jumped out of nowhere and lost it's footing on the road right in front of me. That would've been worse than hitting a car!