What is your age and general fitness level? Would carrying a 20 lb bike (pretty light but not crazy) be too much? If your area is kind of flat and you’re kind of fit, I’d get a nice steel single speed like the Wabi classic, I think it’s like 19 lbs. It will have less maintenance than a geared bike.

a “full sized lightweight bicycle” is normally a nice carbon bike which you don’t need as a first bike, but you can get some cheaper metal bikes that are still light. What you probably want is some sort of flat bar hybrid but those are normally more like 25 lbs unless I’m mistaken