How do I create a page template after the fact?

My website has been up and running for a few weeks now, using a 2011 child theme. I’m pretty happy with the page style, but I’d like to change the post style. The posts go on my blog, which is just one sub-part of the website.

The pages are designed to just have one column. Is it possible to make my blog posts have a 2-column template, without dismantling all the styling that I created in order to design my pages? I’d like my blog to look more standard, with links along the right hand side. Can I turn my page design into a page template that can stand separately from my post template, without having to rebuild it all from scratch?

Please forgive if this question has a stupidly obvious answer. It’s been a few weeks since I built my site, and all the stuff I learned to build it has fled my memory. Thanks for any advice.