An healty young man can easily pre consolidate at 6cm with a lenghtening of 1mm/day without taking any pill. Actually your real goal during distraction is just to lenghten slow to allow soft tissue to grow enough. The real problem is soft tissue, try to make the bone heal faster can be dangerous during distraction, you could not reach your original height goal.
In addition take pills months before has no utility, if you have too much calcium or Vit D you just eject it. These integrators during distraction can be useful only if you have problem with bone growth, otherwise is just a risk of preconsolidation.
Even doing stretching is not very useful actually, i mean if you just stay in the bed and do something you're not improving consistently your tissues flexibility, this is a quality due to your age and your kind of muscle for the most; rather it can be very useful to play a sport, expecially going to the gym to improve thigh muscle (if you are doing femur) cause with a bigger muscle is definitely easier to stretch it, even just running or bycicle can be very useful for it.
There are not magic pills guys, everything require diligence, if you have never played any sport and you are planning LL probably it's better to begin!
Those are pills you can normally take if you have osteoporosis, they can be just be helpful during consolidation to make the process faster.