You can knit blankets in a tube, as you are doing. It just required a larger cast on number.
Since the tube is width you want, just keep knitting around and around until you get the desired length.
The blanket will be twice at thick, making it extra cozy and have no wrong side.
When you cast off, you can seam up the the open ends. You can even seam them a few inches from the end and a little tighter to give it a slightly ruffled edging, if you want a more decorative effect.
Otherwise just seaming the ends together will give you a nice plain blanket.