Seriously, walking counts! Although some might not consider it cardio, a brisk 30 minute walk can do wonders for your mental health, think of it as "mental cardio".
RUNNING TAKES STRENGTH
What I mean by that is, don't just start running and give yourself shin splits like I did. I have done Couch to 5k and HIGHLY recommend it. Now, I'm a runner, something I never thought I'd be able to say.