I am a 31 y/o woman in excellent shape. I am training for my fourth marathon and this year I am training for an early fall marathon for the first time. I usually train for triathlons during the summer season. I am finding it extremely difficult to run in the humidty. I train with four others and we have bumped our starting times to 6:30am to avoid the extreme heat and sunshine. For the last three long runs, I feel extreme nausea, tiredness, and slossing fluids in the belly. The first time, I figured I wasn't hydrated enough and didn't bring enough water and gatorade, so for the second long run I made sure I was hydrated....well for that run I decided to try GU gels as I was feeling terrible again ( I normally take GU in the winter and have no issues) but they made me feel so sick, I had to slow down my pace considerably. So for the third time which was just this past weekend I decided to try eating an hour and a half before the run (previously and for all winter training I eat 2 hours or more before the run) and made sure I was well hydrated and I started the 32km run out strong and feeling great, by 19km my stomach was so upset I had to dash into the nearest bush. I was out of water (I drank 1.5liters from my camelback) and we made a quick stop to get some gatorade. I drank most of the gatorade and tried to keep up a normal pace but I started feeling nausea and weak. By 26km my legs were feeling really heavy, I was getting the chills and the gatorade was sloshing around in my stomach. I had to call it quits at 29km and just felt terrible. I went home, I lost almost 3lbs, but my stomach was so distended. After I ate something small, I felt so much better.

Can anyone offer some advice for me on what I should do? I want to be able to train for this marathon, I am hoping to qualify for Boston 2008. But the way these runs are going in the humidity, I may have to quit or consider doing a half instead.

Just a FYI, in the mornings before a long run I drink a Chocolate Boost and will have 2 slices of toast with pb.
Our average pace for the long runs are about 6:00km/min (in the winter it is between 5:20 and 5:45) and we are running for 10minutes and taking about a 1 minute break. Every break I take some water (in the winter I hardly take in any water). Can overhydrating cause these symptoms? I find I run out of water a long time before my running partners do and they are men and much larger than I.

Any advice would be greatly appreciated.

Thanks,
Z