I have been playing rugby for over 10 years and fitness can improve your performance a hell of alot.
Firstly you need to specific to your position. For example, training for a front row player would differ to that of a wing or fullback. Be specific to the requirments of your position.
You must then identify what area of fitness you wish to improve and design a routine with the specific aim of improving that compontent of rugby fitness.
I play centre, 12 or 13. For my position i need speed and agility in both attack and defence. Strength and exlplosive power for taking the ball into contact situations and sprinting with ball in hand and a good level of endurance inorder to give 100% for 80 minutes.
At my club, we do the WRU (Welsh Rugby Union) fitness tests which are:
3000 metres
40 metre sprint
Bench press 1 rep max
Squat 1 rep max
Bench pull 1 rep max
Max Pull ups
In pre season training, we do alot of conditioning work which involves alot of cardiovascular training and stamina type drills such as going on team jogs (in groups of about 5) with a tackle bag. You each take turns of jogging with the bag. We usually jog for around 5 miles. This would be followed by speed endurance work. This year we did 300 metre sprints, 8 sets with about 60 seconds rest. Following this would be our skill work.
Other exercises for endurance include: Starting on try line, run to 22, back, 10 metre, back, half way, back, 10 metre back, 22 back, try line to try line.
Repeat for sets.
New Zealand Shuttles etc
In the weight room most decent training programmes for rugby will be based around Squats, Bench presses, Cleans, Hang Cleans, Push press, Shoulder Pressess, Barbell Rows, Chins and alot fo core strengthening exercises. Stick to 3 sets of 3-5 with a heavy weight and follow that with 2 sets of 5-10 done very quickly with a lighter weight.
Plyometric Training is also very useful for a rugby player since explosive power/speed are essential for a rugby player.
Say for example you wish to improve your strength and power then an example of a weeks training could be as follows:-
Monday - Weight training (Lower Body) Followed by plyometrics
Tuesday - Rugby traning with club - fitness followed by skill work
Wednesday - Plyometric training
Thurdsday - Rubgy Training with club - skills and team runs
Friday - Weight Training (upper Body)
Saturday - Play Match
Sunday - Recovery day
This would be a typical routine during the season. Pre season you could get rip fo the game on saturday and train both sat and sunday and rest wednesday.
Hope this helps. All the best.