Here is the full list of all 12 Running in the Y2K achievements.
Kill 50000
What are you running of??
Kill 10000 animals
Run a total distance of 40 Km
C'mon Jaytee you can do better than that...
Fully upgrade all the elements on your cave
Kill 1000 animals
Run a total distance of 10 Km
-. .. -.-. . / .--. .-. --- --. .-. . ... ...
Jump on 3 animals consecutively without touching the ground
Kill 3 animals with one spear
.-.. --- .-.. / ... --- / -... .- -..