Harder than implementing RSS. Less hard than writing Linux from scratch. Where it lays in the middle depends a shitload on your requirements. How much you want to federate (a blog post VS conversations for instance), do you try for exact compatibility with existing services VS using a different activity type, is it one way or bidirectional, how does it need to scale, etc. I don't know of a generic library you can use so you'd need to read and understand the spec.
I wouldn't be so sure about that. RSS is a ridiculously complex standard and almost nobody gets it right. Good chance you'll be back over and over to fix bugs and then, when you have more data in your database, you'll be doing it all over again after refactoring things to improve performance and causing more bugs.