Just keep putting one foot in front of the other.  Your body will rebel some this first week, but keep at it.  You are conditioning your tendons and muscles to a new activity.  After a while you'll find that you feel better after you've gone for a little bit.  Normally when I start (on week 7 starting tomorrow) I'll have the aches and pains in the knees and hips, especially if I forget to take the motrin ahead of time.  But about 3-5 min in I start losing that pain and start feeling stronger.  I remember on week 1 I was totally gasping for breath after 30-45 seconds, and now doing 25 min's.

My inspiration is every time I reach a new milestone I get an instant mini high almost, and go tell everyone in the house.  I'm probably kinda annoying.  Plus I feel so much better after I run, I definatly feel the difference on the days I don't do it.  And just making myself a goal, like signing up for a race around the time I graduate the program is helping me keep my eye on the prize, no slacking allowed.

You mentioned once you start you tend to quit quickly.  Why do you start?  What is it that you want?  Those are your motivations.  Keep in mind the long haul,that you won't go from couch potato to fit runner overnight.  Don't give up!!

  My blog:  http://diaryofanaspiringloser.blogspot.com