Yes you can go through greens like water if you train them all...I rec' pick your very best guy, at most two to start with (I did a D and an ST when I started)
Choose a young fast trainer (often teams have one or two to start with, when you run them all fast guys will stand out for sure)
Dont use the stretching, just don't. On my no buying teams I try to avoid stretching, the return for injury risk not worth it.
Always be building better facilities to lower injury risk (pitch is *important*) and to generate reds and greens (no need to speed up building, just keep building) Decide how many greens you are going to use, and figure at least a red per five green, though your results will vary depending on your facilities.
So, what I do is after the day's matches are over, is cardio the PTers at hard, replenish condition, and then do practice match at hard (on good trainers a green equals a cone), replenish..and keep coming back every so often to do it again, until I have gained my goal of skill points or ran out of greens.
and let them rest, don't play them if you are hard training them.. again.. injury risk.