Good steel weapons are not as heavy as you'd think, because you can make a high quality steel fairly thin and strong. Japanese steel was extremely hard and rigid and armor was rare, so it being "brittle" (compared to soft steels) wasn't a problem.

As long as you're not swinging it like a sword a metal rod, or blade, won't feel much heavier than a quality staff or polear..

That being said, making a long ass sword to use as a spear instead of just making a spear would be weird as hell, and I could only see it being done as some dick swinging contest.