Rest for the wicked (ly stiff)?

Questions to all you actual runners out there in blogger land.

How many rest days are you supposed to have a week when you run?
I'm not hard core - I run 3 days a week, every other day, ~6 Km (well walk/run 42-50 minutes) and feel great. I leave 2 days off in a row once in a week, as recommended to me by the learn to run program I was in as they said when starting up I'd have less chance of injury if I took it slow. And I did....but now I'm sort of up to speed, and the running program I'm in doesn't exist anymore so I'm confused.
The thing is, if I were to say add some yoga or my exercise ball workout tape into a couple of other days a week, how many rest days should I still have. Do I still need 2?
I don't want to go crazy or hurt myself doing too much...I feel like my upper body could use a bit of workout is all, since I stopped doing my exercise ball workout when I began to run my muscle tone and strength aren't what they were.
BUT I want to be sure I have time to rest and heal so I can do this forever. I want it to be fun and not hurt.
Anyone?
Should I still take 2 days off?
Just one?
Am I obsessing and it really doesn't matter at all?
Help me out here :)