Short bouts of high intensity cardio (like Day 1s and Day 3s) benefit cardio conditioning much more than 30+ minute moderately paced runs, bikes, or elliptical sessions. But the reverse is not true, i.e. running for 45 minutes at a moderate pace won't help much when you need 15 minutes of balls-to-the-wall intensity.
But by all means, experiment and find out what's good for you. For abs, I found great results doing 8 rounds of tabata leg raises from a bar. e.g. hang from a bar and raise your legs to form an 'L' (or just raise your knees if an 'L' is too difficult) and hold for 20 seconds, then rest for 10, that's 1 round, do 8 rounds.
Also if you want to add cardio conditioning, best to do sprint intervals. There's one posted here somewhere but basically it's like 1 minute of sprinting as fast as you can followed by 2 minutes of a light jog, repeat for as many rounds as you can stand.
If you haven't worked out before or in a while, many people find the pull-ups to really give the abs a workout (especially if you don't normally do pull-ups) so keep that in mind.