The other guides are great IF you just want to grind....but it gets SO boring after a while.........
Here is a much more productive way to "grind"............
What I did in the beginning of the game was ONLY the two missions from the Ranger Station, then I just RAN to every single cabin in the north area of the map, clearing out the Zeds and grabbing all the goodies to store at the Ranger Station...
Reason I did it this way is because I was able to level up my Fighting, Powerhouse, and Leadership, as well as my cardio all at the same time!
On a different note: Its also a great way to level up Maya while you are at it because you don't start to get fatigued until AFTER you make it to the church in Spencer's Mill