Fitness is something that is very important in football, as you know if you aren’t fit enough you wont be able make enough of a difference, and that is a guarantee.
What you have to do is keep it in perspective though.
If it is entirely required for your squad to focus fully on fitness improvement, keep in mind something called the fatigue factor, your players body will indeed do its job in terms of performance when fit, but it needs time to recover during a fitness routine.
Maintain the philosophy of short and sharp, do not let any fitness session last for more than an hour. Minimum breaks allowed and maintain a high intensity level. Including these protocols (you will be hated for it) will see an excellent result.