The last 3-4 times I have run half-marathon or similar distances, I have experienced the preliminary symptoms of cramps around the 19 km mark. It comes as a suddenly "bite" in the upper part of my calfs - it doesn't hurt all that much, but there are no doubt that is will if ignored... When the "bite" comes, I have to run more on my heels the rest of the race to avoid the "bite" develops into full-blown cramps - kind a like "running calf stretch" and not very comfortable.
The "bite" is usually first in the left leg, but after an additional 2-300 m, the "bite" is also present in the right leg. The faster the pace - e.g. during races - the earlier it will happen... If I stretch and walk for a few minutes when it happens and then start running again, the "bite" is sure to reappear again after 8-900 m...
Except for the night after one of these longer runs, I usually don't have cramps at night.
The accepted answer to Why do I get cramps all the time? sugests that it is a electrolyte imbalance and the problem is lack of salts like potassium, sodium, calcium, and/or magnesium. Lots of other articles - e.g. What are Cramps? How to prevent Cramps to mention one - sugest the same, so I can easily accept that.
But... what can I do about it? I really want to running these longer distances...
Are there anything specific I can eat or drink?
I usually eat lots of bananes (which contains potassium AFAIK), and have also consumed large amount of "Tonic Water" (which contains quinine), but neither of these seem to be enough.
Are there any specific gels (or whatever) that could help during the runs? Eating lots of bananes during a race, does not seem to help at all.
EDIT A little more background:
My average milage is 30-40 km/week - less in the Winter (now) and more in the Summer. I have run HM races 5 times in the last year and 17-24 km training passes 6-8 times in the same period. So, I should expect my general fitness as well as my LSR training to be OK.
The last training pass (a rather fast 18.8km yesterday) actually went pretty well - no bites - using a "MAXIM GEL Drink" at 8 km and plenty of Powerade at 5 km and 13 km. The training is part of Sparta Marathon Training leading up to Copenhagen Marathon in May. The training is a little short to be conclusive!!!
UPDATE (2012-01-28): I have begun to run longer during the week and - when that is not possible - use my bike to and from work. That has helped: I have not experienced any cramps since mid December. Thanks...