Tom - better than alcohol with no running!
Ah Jovi - I see. Didn't realise about the controversy.
Oliver - you've come to the right place
. Yes you are too despondent too early. 
1. Never, EVER give up. Long runs - however long at whatever stage of the training programme - are always hard and most of us have wanted to bail many a time, but somehow, if you keep going, they DO end and you'll be glad you struggled on to nail it. Also, if you can manage long runs at the moment on your own - even if they become vile - you'll have no problem running on race day surrounded by other runners in the same boat, crowds of cheering people willing you on etc.
2. Are you trying to run too fast? At this stage long runs should be dead easy. Time on your feet. Do the distance, don't worry about the time. You're building your engine (boosting your endurance) at the mo. The time for doing a bit of engine tuning (running faster) is in a month or so. Take a chill pill.
3. Are you taking in enough energy/water on long runs? Running low on energy will make you feel rubbish.
4. It could be that if you've upped your volume over the last few weeks it's beginning to wear you down a bit. This could be a sort of 'it'll get worse before it gets better' thing and any minute now you'll start to feel stronger. Or it could be that you've been doing too much too soon - in which case back off a bit. Keep the frequency of your runs up but have a week where you don't increase your volume/speed or anything to let your body catch up with itself.
5. Don't worry about how far your training still has to go - it is too daunting for most mortals. Just focus on each run in each week as it comes. If you just follow the programme you'll get there. Promise!
6. It might just have been a one off crap run. We all have them.
Just a few things that come to mind. There could be more, but stick around and you'll see that we all go through rubbish periods and doubts. The great thing about marathon training and following a thread like this is that it really spurs you on when you see people talking about the sessions they've done. It makes you (well, me anyway) look forward to the time when I'm posting the session I've done.
Here endeth the lesson. 