I've been running for around 7 months, and my longest run so far has been 15km. I went out yesterday for a trail run with an experienced partner and we planned on doing around 18km - running the flats and downhills, and walking the uphills. We inadvertently took a wrong turn and ended up running/walking around 25km, it took around 4 hours. I was really pleased with how well I did - I felt great for most of it and by 20km I was still running comfortably. Obviously I'm a bit stronger than I think I am. Towards the end, I found it was more painful on my legs to walk than to run, but I was pretty tired by the end and walked the last km in spite of that. About half way through that last km, from my hips to my toes was excruciatingly painful (so bad I was nearly in tears), it was hard to walk at all, and I felt dizzy and lightheaded and thought I was going to throw up. I got back to the car, had a drink and a stretch and felt better very quickly, the pain went away. The rest of the day was uneventful, took the dogs for a walk for an hour, and today I feel fine, legs are great.

I'd just like to know how I can avoid that pain and nausea/dizziness next time. A few things come to mind but I'm sure more experienced runners have some other ideas, and I'd really appreciate suggestions.

I probably didn't take enough to drink with me - two 700ml or so handhelds with dilute electrolyte in it. I was rationing it, and by the end was thirsty. Maybe dehydration contributed.

Not enough to eat - had some jelly snakes to snack on but not much else. Breakfast had been tea and toast early in the morning and that possibly wasn't enough.

Electrolytes? I was drinking half strength gatorade - I don't like it full strength, and I hate the taste of plain water, maybe not enough electrolytes for 4 hours on the trail.

Thanks for the suggestions, and please, no flaming - I know that I could have carried more water/food, but I was guided by my running buddies.