Help!

Today my legs gave up! They finally just cut out on me like an old battery in a car. I was on a planned 18 miler, admittedly it was a tough 18 miler, undulating with some testing hills around 12 miles. To my surprise I got up these hills but they must have sapped the last of the energy from my legs because come mile 15 I just stopped. I've never felt like this before, but I mentally and physically could not go on. I walked for a couple of minutes and tried to go again but within a few hundred yards I was forced to stop again. This was the pattern for the remainder of the run walk! albeit the running part was getting less and less. I stopped the watch at 15.75 miles I couldn't include the rest as part of the run because it wasn't running.

The worrying thing is that recently the last couple of long runs have gone this way and I have a tough Marathon approaching in 8 weeks time. I'm not sure of the problem, but this is where I need people's help! Any advice big or small would be much appreciated, please leave a comment.

I'll give you a little bit of recent history. You can see below my running for the last couple of months. Have I been doing too many long runs? Not enough of something else?

My second concern, Nutrition. Today's run I took my Camelbak filled with water, so there was plenty of water on tap! I also tried a new gel. The SIS Burners gel, these work in a slightly different way than a usual gel, The theory behind the gel is that the added carnitine in the product increases the levels of carnitine stored in the muscles so you become more efficient at burning fat as a fuel and therefore helping to spare your carbohydrate stores. Gels just don't seem to give me any benefit and this one was no different. I took it half way round, and the only thing I noticed from it was the bad after taste in my mouth. So if Gels aren't working, what else can I try, filling the Camelbak with a sports drink? Ideally I would like to go on a long run without the Camelbak but a 330ml sports drink is not enough for me for 3 hours running.

I actually don't mind running with the Camelbak, it's a great product and after a little while you soon get used to it on your back. I have tried running with a cereal bar. I found this great (real food!) but just not enough to keep me going.

I'm getting concerned. As I was walking at the end of today's run, thoughts of  'Maybe I'm not a Marathon Runner' filled my head, but I remind myself that apart from the last couple of runs, I've been out for 18-20 mile runs and have been OK. My thoughts are that maybe I'm making my long runs too hard, they do tend to be undulating.

I would really appreciate any help on the above in my quest to get me back up and running.

Thanks in advance.

Ian