First Tri!

Welcome aboard!  And "Ehhh why not?" is a VERY valid reason to take up tri. 

Having some structure will likely, IMO, be a good thing for you, as it makes easier when you're getting into training - less time wondering what you're supposed to be doing and such.  Always important to match the effort and type of plan (beginner, intermediate, etc.) to your goals.

As for short/long, without knowing the plan it's hard to say exactly, but I'm guessing just what it sounds like.  Either with regard to distance or (more likely at the beginning) length of time for the workouts.  Do what feels like a short run on a short day and long on a long day.

That said, I think you'll find that just a little more specificity can be helpful.  Once you target a tri distance (sprint? Olympic? etc.) and how much total time you want to devote during the week to training, that will help scope the plan to your goals better, if that makes sense.  IOW, it'll help define what "short and long" mean to you, at this point in your training.  Sorry this is a bit general, but sounds like you're just getting started, so general might be helpful.  Happy to be more specific if you have specific questions...

And keep asking!  The more you ask, the more you learn.

Have FUN!

Matt