I often leave with half strength and it seems to work for me. Check your urine, often. Mine is usually clear for the first 50 miles and then it tends to cloud up some. The temperature has much to do with the amounts of fluid you should consume but, even cold days will dehydrate you as will elevation. I find that I drink more when carrying the bottle in my hand and not in the pack.
Food is a more interesting subject.
You have to have it. No food, no go. At Arkansas I had planned to have Metabol at every drop. After 30 miles I realized that I couldn't force it down. I had opted for soup and carried it in my wide mouth bottle.