It's amazing to me that you're 23 and not lifting weights...especially if you are playing rugby.
I guess it's a different mindset here in America, but they start us off at 14-16 lifting for all sports (football, basketball, baseball, ect.).
Yes, you're big, but what if you came against me? I'm 6'5 too. But, I lift weights...so my mass is increased by strength...lifting weights gives you the ability to do the most with what you have.
Focus on your core (abs and lower back), your legs, your upper ches and upper back (arms are tied into the upper body...back and biceps, chest and triceps).
Start off simple:
Bench Press
Back Row
Sit Ups
Back extensions
Squats
Lunges
Hit the treadmill at it's max incline for 10 minutes at 3.2 mph (don't know what the kilometer equivolent is) to warm up.
From here you can add things like variations of arm curls and tricep extensions...and leg curls and leg extensions.
If you hit the weights you will see an improvement in your game...it's that simple...the stronger you are, the better you are.
Oh, and also take the advice of playing with your head as well...out smart rather than out muscle sometimes...then you can save your strength for when it's really needed.
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