As an addendum to the very good post by jams - consider the effects of terrain on your movement as well as the obvious "cover from shooting".
If you place an objective on the far side of terrain (such as Player B has kind of done in the last example) then it can easily double the time it takes to reach somewhere. Don't assume that all your transports/bikes etc are going to survive, and plan to be able to reach any objective with foot infantry (esp. SnP models, or models that otherwise don't want to run) by turn 5. That means if the obj is over 12" away with a terrain piece between you and it, you want to be moving turn 3 at the latest, because you're only averaging 6-7" of movement per turn instead of 9-10" as you would in open ground.
Also consider that closest models are removed first, so not only do you need to get a single model within 3", you need to get enough models to survive whatever shooting the enemy has left and still remain within that distance. I've seen several people make this mistake, even this far into 6th Ed.
Indeed. Wise words to remember