Typical people can absorb about 1 liter of water per hour. You don't need to start 24 hours in advance, but you do need to start in advance. If you start up "behind the curve" then go out and do something that loses a lot of water, you will usually stay there. Drinking during exercise will help
keep you hydrated, but it's a losing game if you start out dehydrated.
Very well trained athletes can sometimes absorb as much as 1.5 liters per hour. Problem is that at the extremes (exercise in high heat/humidity) you can be lose more than your body can absorb. Most of us will never work out at sustained performance levels where this is an issue. It's something marathon runners, adventure racers, triathletes, etc. need to worry about.
Sugar does nothing to promote the absorption of water. Neither do any other additives, nutrients, etc. There are good reasons to add salts and other electrolytes to the water if you're drinking and sweating a lot over an extended period, but they are not about hydration, they are to prevent something called exertional hyponatremia:
Exertional hyponatremia: Too much of a good thing — Human Performance Resource Center This is partly the result of the "you can't drink too much" mentality that is so prevalent these days.
Urine color is the best "quick check" for proper hydration levels. Clear to a very light yellow and you're pretty good to go. Darker yellow, and/or haven't needed to go in a few hours and you're moving towards dehydration. Clear with repeated need to go and you've probably taken in more than your body can absorb right now. I rarely go for more than an hour or two before stopping, so I just pick up a bottle of water and drink it when I stop.
