Depression affects your self belief and confidence so when you feel like something might be aching or its getting hard your brain says "ok, you can't do this, it's too hard, time to give up". Where I live is surrounded by hills. Some days I seek them out, others I opt for a quick jog in the park because that is all I feel I can do. On more than one occasion I've failed to even get to the top of our road (I live on a hill) and decided it wasn't worth carrying on. Yup, depression is a pig to running. I'm sorry you're finding it so tough. Set little manageable targets. Your past achievement aren't there to beat yourself up over. I've run marathons, but I'm not at that point in my current cycle so I'm not aiming to go out and run one tomorrow.