Which one to do? Diversity is the obvious choice. The best approach is one that challenges your body, and forces your body to adapt.
You want to mix Medium intensity with High Intensity Training
Most sources recommend 3-5 days for cardio.
If you cant already run, check out a Couch to 5k running routine, like the one below, it'll get you to run 3 miles even if you cant make half of one now.
Once you can do some running you need to decide on some longer distance and some shorter distance days. Determine these days by choosing the days you're most sore to be further from your leg days (weight resistance training), or allow your legs to rest between working them. Up to you how it goes.
With longer and shorter days determined, now you need to decide on the intensity.
Shorter days. On these days you want to incorporate sprints, hills, and hill sprints to your run. These short runs will give you the ability to run at higher speeds for your longer runs, better endurance, develop them hip flexors, and recruit your fast twitch muscle fiber. These days can even help with increasing your leg lift weights.
Longer days. Shoot for a longer, sustained medium intensity level through out the run. Sprint the last half block - block, if you feel up to it.
Eat something before the cardio.
First week: Run for 1 minute, walk for 1.5 minutes. x8 (20min total) 3 days that week
Second week: Run 1.5min, walk 2min. x6 (21min total) 3 days that week
Third week: Run 1.5min, walk 1.5min, run 3min, walk 3min. x2 (18min total) 3 days that week
Fourth Week: Run 3min, walk 1.5min, run 5min, walk 2.5min, run 3min, walk 1.5, run 5min. (21.5min total) 3 days that week.
Fifth week: Run 5min, walk 3min, run 5min, walk 3min, run 5min. (21min total) for the first day. Run 8min, walk 5min, run 8min (21min total) for the second day. Run 21min for the third day.
Sixth week: Run 5min, walk 3min, run 8min, walk 3min, run 5min. (21min total) for the first day. Run 8min, walk 5min, run 8min. (21min total) for the second day. Run 23min for the third day.
Seventh week: Run 25min for the first and second day. Run 28min for the third day.
Eighth week: Run 28min for the first and second day. Run 30min for the third day.
Ninth week: Run for 30min 3 days that week.
The specific run/walk times (1.5 min. 2.5min, etc) are a pain in the butt. but. It works! (btw this is from the book Show Me How)
Don't forget to warm up and cool down! I do 10 minutes of walking for both and some body weight squats.
Rest a few days in between your workouts
If you feel more confident than week 1, start where you like. I jumped in at the start of week 5.
Run for time, not for distance. (Never mind the "...to 5k" part, that should be "...to 30 minutes" :P )