As the user below said, muscle gives tone. Cardio activities don't necessarily build muscle. Cardio is always good but if you are not looking to lose massive amounts of weight/bodyfat, which you are not, then you don't need much. I'd say no more than 20 minutes a day if accompanying a resistance training regimen, and yes that should be your primary focus as far as exercises go.
Tone is muscle with little fat. Get some strength training in, THEN do cardio. Do strength before cardio in a workout for 2 reasons. 1) pre workout cardio will hurt your strength performance; 2) your body continues to burn energy at an elevated rate even after lifting/strength training, so by doing cardio in this window, you add on to this effect.
Strength training, and you need to triple your calorie intake based on what you've said elsewhere. You will not 'tone' on 800 calories/day.
More muscle. more tone.
If you wanna tone, you're probably gonna have to do some strength training. But you don't really need to go to the gym. You can do squats and lunges at home and maybe use something like a textbook (lol) for arm curls.
The key is to do a lot of repetitions; anywhere from 14 and up if you wanna tone. Weight isn't that important when toning.
Muscle gives you tone, so build more muscle.
Running burns fat, and since you're already slim that doesn't do anything to build your shape.
You want tone, run run run run run run run and the...run some more lol. You want muscles then you hit the gym. But if you want to slim down then save yourself a membership and hit the road hard. You get out of it, what you put in.
To tone it's better to do specific training for the part you want to tone.