Definitely plugin man. Interlinking manually will be such a huge headache especially when you are planning to grow your website.

Use SEO Smart Links, the best plugin for the job.

Before you judge me, i have never used a plugin for it so i dont know what functions i have.

But it is rumored (by MOZ ) that you should always try to link to the oldest possible content from our newest, and only ever send one interlink to an old post. nless ou are cornerstoning, but that isnt really viable all the time.