The link below is from Goldfish, a very good generic trainer on these boards.
http://training.fitness.com/young-athlete-development/young-athlete-development-program-50627.html
I will add a couple of warnings based on what you have said here.
Don't go for uneven training based on strengths, occasional compensation to bring weaknesses up to form is one thing but leaving things out because they are strong soon results in them becoming weaknesses. If you need to know why not to leave out legs day google image 'never miss leg day' for a bit of a laugh.
Obviously you are wanting to get into better condition for rugby, meaning you need more endurance and stamina to keep your mass moving for the entire game while keeping some explosive power to 'relieve' another of the ball or keep it from being removed by sprinting. You have stated 12 weeks as a time frame and the reason is a good one. Try to keep this as a short term goal to get you started but ensure there is another ready for when this time frame is over, then another etc. In other words treat this as a lifelong not a short term change.
That said ensure you change the programs regularly to keep them interesting and effective. Normally I do this every 8 weeks, this could be as little as 6 or as much as 12 if you want.