You shouldn't have to worry too much about getting a seat. Economy awards unfortunately do not allow advanced seat selection on CX. However, as mentioned above you can select a seat online 48 hours prior to your flight departure time.

CX blocks a lot of seats and many other passengers are in the same boat as you, without a seat assignment. So as long as you check in online around 40-48 hours prior you should be able to score adjacent seats or window/aisle no problem. I have not had a problem flying them many times.

Additionally, if you don't see the seat you want, you can keep checking back as people will move seats / change flights etc before check-in. So just make sure you don't actually check in, just change the seat online.