Discouraging discouragment

I'm running on Friday for my marathon training. I've been running for about 7.5 miles. At this point I'm running slightly uphill and the wind is pretty strong and I'm running into the wind. From what I planned on running I'm only about half way done with my run. Well this point I feel like I can't run anymore. Seriously felt like just collapsing and crying on the grass I was running by. I just wanted to give up. The wind continued to blow harder so that I was jogging but I actually felt like I wasn't really going anywhere. I force myself to run a mile and a half longer. I am stillabotu 2 miles away from home. I walked those 2 miles. Not really glad that I crapped out when I've gone beyond 7.5 miles before. I don't know what happened but I was very discouraged and upset with myself. With that sort of thing happening I'm not sure I'll be able to run a marathon... sheesh... ugh well gotta carry on somehow.