Cracking 20 Digit Uppercase+Lowercase+Numbers

[marc1n]

Hello,
Of course, I am an absolute newbie here. I am trying to crack one of my wifi WPA2 passwords by brute force using my GPU. Already have the 4way capture taken to hccapx. 

The part I am missing is how I should compose the brute-force attack if the password is a 20 digit one and can have: uppercase, lowercase and digits. No special characters, nothing else. Just letters in both lowercase and uppercase plus digits. Might be all uppercase or all lowercase.

Any recommendations on how to set this custom charset?

I appreciate it in advance.

https://hashcat.net/wiki/doku.php?id=mask_attack