I think just give them friendlies.
In preseason I don't think fitness means anything more than training their phyisical attributes mostly. All that about injuries and stuff is a myth (I haven't done anything serious to say it for certain, but I've not experienced any difference, I already had few injury issues before I started to try the fitness in preseason, in fact I think it just increased my injuries during preseason). The one thing that seems to have helped me get them fit faster is that instead of changing the eleven on the field by halves, I do by matches. I choose alternating starting XI's and I don't do changes unless forced by injury.
The, during the season, I make sure to have a reserve team friendly one or two days after each first team match so players that I don't use enough get some playing time.