Veronica, I have the same question! I just checked out the BeginnerTriathlete (or, beginning triathlete?) website.

There are a variety of training plans to look at (for free). There is also a good article about choosing a running training plan, and it discusses what to do if adding running is the issue (including if there are old injuries, etc).

There was a discussion of how to choose distances that took into account the level of endurance training you already have done in another sport.

I have never done a tri, but I am intrigued, and I found good info on the site.