Getting your nutrition and electrolytes right is one of the hardest challenges in endurance events, and one of the most important for successful completion on the course.
Mentally fuzzy? Nauseous? Fingers swelling? Feel especially cold after the race? Lack of sodium is probably your culprit. Most people take in way, way, way too little sodium on marathon courses. They rely inappropriately and mistakenly on negligible sources of sodium like Gatorade and pretzels and sports gels. It's even more critical to get your sodium replenishment correct when you've already done a 2.4-mile swim and a 112-mile bike earlier in the day.
That's also why experience helps so very much in these long-distance events. You only truly know after you've completed numerous events what your own particular body needs in different weather and exertion conditions, and how much sodium and fluids and how many calories to replenish along the way.
For me, doing the two consecutive marathons in very similar weather conditions was extremely instructive. At Baltimore I took 12 Succeed caps (4092 mg sodium), which was a bit too much, based on uncomfortable urinary symptoms afterwards (um, burning pee, if you must know). At New York, under similar ~70*F weather conditions, I took 6 Succeed caps (2046 mg sodium), which was not quite enough, based on a little bit of finger swelling I experienced on course and the nausea which I had afterwards. I didn't bonk and I didn't experience substantial mental fuzziness or dizziness at either marathon, in contrast to some of my earlier attempts at the distance when I took in much lower levels of electrolytes.
That information is worth SOLID GOLD to me in tackling an Ironman next year. I have narrowed down my requirements for sodium fairly precisely to somewhere around 2500-3500 mg for every 6 hours on course, or ~500 mg per hour. That is money in the bank for me which will help ensure a successful completion of Ironman Florida in November, 2006.
I may not be able to train this body to be very fast within one year, but I can certainly do everything I can to get the nutrition part of the Ironman equation correct. It won't make me faster, but it's one of the most important things to help prevent me from slowing down or stopping. One step closer to my goal!