Pace or Distance?

While I am training for this crazy half marathon and just really trying to get myself healthy and improve the quality of my life, I am noticing that I am (no surprise here) way too hard on myself.  I know I have posted about my conflict on whether to run or walk but what I am now questioning is, is it better to improve your speed/pace or go longer distances?  I've noticed my version of walking is faster than most and my version of "jogging" (I would never go so far as to call myself running) is a lot faster than others.  But when I "jog" I cannot go as far as when I "walk" at a faster pace.  So which is better?  Help?