Even though Marriott's T&Cs say their points expire (and this is only based on account inactivity, not the time since you stayed in the hotel) - however in actual fact they don't apply this rule. So, you don't need to worry about it - your points won't expire and neither will your account.

However, if you are being ultra-cautious, just buy something through the Marriott mall to earn a few points, this will keep your account (and all of your points) active for another 24 months.