Well, you can travel very light if necessary. For a day hike it's not really a problem - in a temperate climate I'd be perfectly comfortable with a very basic kit:
- knife
- small survival kit (SAS tobacco tin type)
- water canteen
- some snacks
- poncho
- woolen sweater

All of this will fit inside a very small backpack. Since you were in an area where water is abundant, carrying 3l seems a bit redundant. Actually, if you have the skills and confidence this same kit would be pretty much enough for an overnight trip, too. Just keep in mind that the less you carry, the easier the walking - but when staying overnight you'll need that much more skill and effort to make a decent shelter and improvise the stuff you'd otherwise be hauling in your gear. So 19lb is definitely over the top for a day hike unless you are in the Arctic.

I do agree though that you should take a serious look at your physical condition. Unless you have some kind of disability a 19lb pack really shouldn't be a problem.