Right... Where to start...
Diet
Have you measured your average daily calorie intake across a week and against your basic metabolic rate (google it and its calculation)? Because what you posted seems great and healthy for somebody that isn't doing any exercise. Eat more of everything.
The reason you're flat at rugby is because your body isn't getting enough anything the maintain energy levels. One of the posters on here is a serious swimmer and wants to add mass, and is therefore on a diet of around 5000 calories a day to support it. You may well not be too short of that.
exercise
I think you will need to reeducate yourself regarding the definition of 'fitness' as all it means is the body's capacity for exercise. Exercise, as you are finding out, comes in very different forms: endurance, speed, power, strength (variants of one or more of the above). If you're adapted to one, the others will suffer simply because the body's metabolism can only really be wired for one or another: not all (heptathletes will always suck in one type of exercise, for instance).
Because you are not used to the exercise regime at CrossFit, your body will take more time to adapt to it as opposed to playing football in addition to rugby. Add in an insufficient diet like yours, and your body will be going into starvation mode, burning both fat and muscle.
This is also where CrossFit starts to fall down as it is none of either strength, speed, power or endurance training. The premise is to create the '"fittest person in the world" - "the jack of all trades," if you will. The problem is that almost all sports require some form of specialist conditioning, and CrossFit will not provide it, instead bunching exercises together to artificially stress the body. Rugby, for instance requires very powerful athletes who can perform repeated sprints over 80 minutes whilst ploughing through tackles.
As such, the work required for rugby players is specific strength and power work, complemented by speed endurance training. Separately. Not together, as in CrossFit, because to do strength optimally, the athlete needs all the energy they have. And vice versa.
During your rugby training, for instance, you probably do loads of endurance work (shuttle runs, etc.) but no strength work: CrossFit will help for a while because you!re not used to strength work, but your body will adapt and not derive further benefit from the mixed workouts they prescribe.
In summary:
- Eat more
- Carry on with CrossFit for the time being
- Think about whether you want to become awesome at Exercising or awesome at rugby.
