Problem is every single enemy will likely just beeline towards you if you get spotted and if they are equipped with full-auto weapon then it only takes a few seconds for you to get killed. A few suggestions:
1. Listen to footsteps.
There's no other audio cue on approaching enemy and your view distance depend on your attachment. The footsteps sound different depending on the area so you can kind of figure out where the enemy is. There are some issues though so you cannot rely on it completely. Some areas produce almost no footsteps sound making it impossible to hear approaching enemy and since it's impossible to figure out which way the enemy is coming from just from the footsteps, treat it more like an alert that an enemy is getting closer.

2. Try to find scope, tactical flashlight and night vision.
Scope will allow you to see further so you can see and shoot the enemies before they see you. Sniper scope is probably the best since you can see really far. Flashlight can help inside buildings so you can see beyond the fog of war. Lastly, with night vision helmet, you can see your surrounding area further as well within the fog of war.

3. Remember to pick up free supplies after every run.
It is near the bottom left corner of the base next to the door. Equip them to your hotkey. The splint is your best healing item (yes it can treat broken bone but it heals a decent amount as well). IFAK is kind of your second chance item since it heals a lot. I like to use it if I get ambushed and lost a lot of health and just use the IFAK to regain most of my health back.

4. Use the knife.
Very OP especially in hardcore since you can just get in without bringing anything and just knife enemies. Most enemies die pretty quickly even bosses.

yeah the knife is really fun too