COPY OF MY COMMENT TO @steemmatt IN YESTERDAY'S POST!
Just thought about copying this here, because I believe that something like this could be a solution to the "cardio problem"! Also, I'd like to get some discussion going! Do you have a better solution?

↓ The comment ↓

I fully understand this! I'd recommend possibly capping the "maximum reps" for cardio to 200 or 300. Also, something that's worth thinking about is making the 100 reps=1 hour of cardio instead of the 30min currently!

This is truly something that needs to be addressed because, as you said, soon there will be those 1,000+ rep days of just walking or cycling... If I started counting the walking I do every day (not for working out purposes), I'd easily have 1,000 reps every day :/. I understand counting walking/cycling as cardio if it's specifically for WORKING OUT purposes (i.e. going on a walk in the evening after work) not just counting the walking that you do every day at work/school!

↑ Something like that might be good to mention in the rules ↑

PS. I personally think that sports should definitely be included in the "cardio" section of this challenge (even if they're just for fun)! Of course, this depends on which way you want to take this challenge, but still, if I play 1h of tennis, it's definitely more intense cardio than walking for 3 hours :D! But the idea of capping the cardio reps is a really good idea in my opinion!