Infected Gamemode.
You are infected. Shop items are 50% more expensive. You start the game with a permanent 10% hp and 10% less speed and dmg. You're halfway to Mexico, you just need to make it 40km. You have a shovel, rifle and revolver, each with 25 bullets.
At 10km the infection gets worse. Your vision takes a green hue, you can no longer sprint, and you have 20% less hp.
At 20km your melee attacks do less damage, healing items heal you MUCH less (60% less)
At 30km, You're on your last legs. Even walking costs stamina. Your vision is blury, and you do no melee damage.

At 40km, you can see the bridge. You shamble forward, gun at the ready.
If you somehow win, you get the Determined To Live milestone: 20 stars and 25 bonds.
Reward for beating the game mode: 50-75 bonds.
Also, you cannot use classes for this game mode.

Alternative 80Km infection mode.
You start at the first outpost. 10% less hp and 10% less melee dmg.
After 10km you get 10% less movement speed and take 5% more dmg vulnerability.
20km: your vision is wobbly.
30km: green hue.
40km: no more sprinting and take 15% more damage.
50km: healing items heal less health (60% less healing)
60km: walking costs stamina and you do no melee dmg.
70km: Blury vision. (Makes guns harder to use)
80km: You cannot jump. (Well, you can jump, just not as high anymore, only like 3-5 studs)