Camping an afk rock crab or sand crab spot would be ideal since you're searching for a way to gain combat experience while doing cardio. I'm not sure which locations are best, so I'd suggest finding a guide on youtube to assist you in which areas are currently most effective for afk training on a pure.
I'm not sure how long your cardio routine is (mine is no more than 30 minutes), but if i'm not mistaken, you will lose agro after 15 minutes at the waterbirth island rock crab afk spots. Just something to consider