Stopping miles from expiring on American Airlines

I have about 35k AAdvantage miles with American Airlines that are about to expire. I understand that if I fly with American this will reset the clock for another 18 months. (Alternately I could spend money to save the points)

But what I am not clear on is do I need to fly before the points expire, or simply book a paying flight with American?

I had intended on using the miles to purchase a trip in 2019, but the number of miles exceeded my balance, and the cost to buy miles to make up the difference was actually more that just buying the flights outright.