I found an easy way to manage SP, simply accept all the missions available to you so you can get more places to visit together, then choose the closest one (this reduces traveling SP cost) this way you can do many missions. plus you can play almost one and a half hour a day + if this helps.
This is impossible to fight a zombie, bare handed. I recommend that every soldier must have a weapon from the start to be more realistic. Other than this the game is amazing