In ACE you can not run forever. Your movement speed, like running, sprinting, rolling, or walking slowly effect how soon you will become tired.
Different classes have different stamina levels: Engineers, as well as Special Forces units, have increased stamina, while medics, crew members, etc. have lower endurance than a normal grunt.
The total amount of gear you are carrying, including ruck contents, has a huge effect on how fast you become fatigued.
The stamina system also includes a certain amount of inherent weight representing boots, helmet, body armor, etc. - roughly 20kg.
Additionally - other influences like weather, wounding, climbing slopes, or suppressive fire have some influence on the rate of fatigue.
Once you have reached your maximum stamina threshold you collapse.
This is likely when carrying a heavy rifle/machine-gun with lots of ammo and any launcher-type weapon at the same time, e.g. when you overload yourself.
Your current stamina level is indicated by sound and graphics effects like heartbeat, breathing hard, and in the worst case collapse and blackout.
In order to regain your stamina, you should rest. This can be done by standing still. The position also counts a bit: prone >> crouch >> standing.