"Tough As Nails": Sore Quads - HELP!

I am a 21yr old female, training for my first 1/2 marathon (October 18th) and am following a moderate training schedule off of my race's website. I've been running for the last three years, but nothing longer than 6miles or so. I am able to do my runs with great energy and my heart/lungs are able to make it, but gradually my quads have been aching more and more with each week. At first, they started to get sore after 4miles or so, now I can barely make it a mile or two before they are screaming at me. I am allowing adequate rest days and "recovery" weeks that are built in to my program. I am trying to do some weight lifting to possibly strengthen my quads, but they are so sore that I can hardly do any lifting. I don't want to give up on my training schedule, but I also don't like running in pain either. What am I doing wrong or what should I do to keep running? I am really enjoying training for this 1/2 marathon and it sucks for my quads to give out on me before my body is ready to stop running! I have never had problems with continually sore muscles before... Any advice?