At 104kg you should be hitting the cardio pretty hard especially for kitesurfing. Work your way up to 1hr of heavy cardio 4 times a week. Dont try and do this immediately as your heart might explode (look up heart explodingitis in the medical journals), you also do not want to risk an injury as it will hold you back months if not for good. As a ex-rugby player I'm sure you know how to build up your fitness... just dont overdo it at the start, build up slowly and stretch heaps and heaps. If at all possible, do one of your cardio sessions on the sand.
Swim at least once a week (make it part of your cardio regime). You are going to want to be confident that you can swim 1.5km back to shore after 2 hours of hard kitesurfing. Swimming also helps considerably strengthen your back which is very important when wrestling an out of control kite. Rowing is also a great cardio exercise for getting a powerful back.
Do heaps and heaps of squats, NOT the kind where you load up the bar with all the weights you own plus your kids, keep the weights low enough to just get out 20 to 30 reps WITH GOOD TECHNIQUE!!!! Never do squats with a poor technique cause you will and up with a bad back or a hernia! Keeps the reps moving from a slow to medium speed never quickly, dont let momentum play a part in the lift. Build up to 5 sets of 20 to 30 reps, but again start off easy and keep good technique otherwise you risk injury. I didnt realise how much kitesurfing worked my quads until my first session over an hour and a half.
Lots of chinups or lat pull downs, with lots of variations in width of grip and the direction your palms face. Again instead of going for pure weight, try and keep the reps between 20 and 30. Try to use your back to pull your shoulders as far down as possible FROM THE START OF THE REPETITION, this will put much more of the force onto your back which is what we are trying to strengthen. Keep the repetitions at a slow to medium speed to reduce the momentum of the lift and to reduce injury.
The same goes for Cable rows as it does chinups. Again pull your shoudlers back from the start of the rep to place more force onto your back.
KiteHard suggested rockclimbing which is also great for back strength and flexability, maybe include it as a cardio session if you climb hard and long enough!
WORK THOSE ABS!!!!! Lots and lots of situps, not much to be said here except keep a gap the size of a tennis ball between your chin and your neck and use a mat. Do situps that you are comfortable with not ones that hurt your back, include leg raises as well.
When you accidently unhook from your kite it is handy to have powerful triceps to get the bar back down level with the hook. Tricep push downs on a cable machine (sorry I cant recall the correct name) is the exact motion you will need to get the bar hooked back in. Stick to heavier weights with lower reps for this one as the motion may only be performed a few time in one kitesurfing session.
In addition to all of this include a demanding stretching regime, as Bondo suggested try different styles yoga a few times a week or maybe pick up a couple of Ballet classes
It is important to be very flexible as it will drastically reduce your chance of injury and most importantly stop most chronic injuries that can lead to persistant problems for decades.
These are the specific exercises I would recommend to get the most out of a kitesurfing season. Personally I would add some dead lifts plus some bicep, chest and shoulder work but only for the look.
First go see a sports doctor, get him to check out any problems with joints or mucles, get a general cardiovascular and respiratory examination and make sure your iron levels are high. If you have low iron levels you will make yourself sick and fatigued within days.
At 104kg you will also need to change your diet to lose weight and to accomodate the extra cardio work! More complex carbohydrates, less fats, no refined sugar, heaps of fruit (introduce the fruit gradually unless you want to have ass-explodingitis), legumes, beans, some red meat for iron BUT replace half of your meat intake with fish, avoid chicken because of the hormones that give men bitch tits. You will find that if you drastically reduce your dairy intake you will have more energy BUT still maintain enough to provide your body with calcium. Replace any wheat in your diet with oats. The last two points are included as much more than 50% of the population has either a wheat allergy or a dairy allergy or both AND they will experience a great increase in energy if they remove those foods from their diet.
THIS WILL TURN YOU INTO THE ULTIMATE KITING MACHINE!!!!!!!!!!!!!!!!!!
If anyone can come up with any other exercises for specific muscles used for kiting that I havent included, please share!!!
Good luck!
plus I will cut back on the firedancing and once of the rock climbing sessions.
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