You can focus on one ZWOW each week or every day for a week or chose one to two different ZWOW’s to complete each day. There’s plenty of variety available and that’s completely up to you.
My Personal Experience:
If you want to work on one particular ZWOW regularly the best way to do that, in my opinion, is through overall fitness so change it up and do different ZWOW’s each time. After a couple weeks or a month, depending on how often you’re ZWOWing, you’ll definitely be stronger and if you want to try to beat your score on one you’ve already done, go for it!