How are you going to play rugby if you can't run more than 20m?
Leaving that aside (or assuming your shin problem is going to clear up), you have loads of options you could implement.
GPP work could be classed as any work you do which doent specifically prepare you for the game, so pretty much any fitness work you do out of practice could be counted (weights, sprints, other anaerobic work, agility drills etc).
I would assume you are also in season at the moment (being from down under) so you want to make sure you are well recovered for game days.
With that said, assuming you have two practices and a game per week, I would say that two weights sessions and one longer anaerobic session should suffice.
As regards to that anaerobic session, it would be useful to know what sort of fitness work you do in practices, but I would recommend some sort of fartlek training ( http://en.wikipedia.org/wiki/Fartlek), though any sort of interval training or circuit training would suffice (complexes are very good for anaerobic conditioning: http://www.elitefts.com/documents/mma2.htm).
I have assumed that you do some sprints etc in your specific sessions, more info on your current training would be useful.
I also played in the front row for a long time, so its great to hear from someone else in that position.
Keep up the training!