How is it that all the wars during the Targaryen era last between one to two years only? Given the size of Westeros and the travel distances therefore involved shouldn’t the Dance, the Blackfyre Rebellions, etc. have been longer?

Yeah, this is a pretty significant worldbuilding issue. Leaving aside Westerosi travel distances, most real-world wars in the Middle Ages and before were pretty long-lasting affairs. Sieges lasted a long time, fighting was seasonal, etc.