In your case, you wouldn’t need much cardio to stay in shape, as it is essentially a calorie-cruncher. Strength exercises with adequate weights are your best bet for ‘toning’ – which essentially means growing muscle and losing fat. Since you don’t have fat to lose, building muscle is your step to more definition. Practise strength sessions 2-3 times s week, with 3 sets of 8-12 reps of each exercise. Your weights should be heavy enough that you struggle, but keep form, on your last few reps. This also needs to be coupled with the correct nutrition, and your post-exercise meals need to be high in protein to refuel and rebuild your muscles.