I would advise you to check out the sliders forum. I currently use adembroski's sliders and the work great for me.
Secondly, you really have to commit to the running game and understand the defense you are going against. For instance if you are playing the Colts who play alot of Cover 2 and little no no blitzing then you can pretty much pound the ball up the middle the entire game, but if you are playing the Steelers who constantly bring pressure up the middle then you need to work in some draws,, some outside runs, and the occasional pitch. When you get that going you can start to work the ball up the middle a bit more because the pressure will be more the outside.
Also you have to think about this game like real football and understand what type of back you are using and how running games really work. Big power backs with good trucking skills will normally be more consistent because they can truck people better and get yards after contact. Quick elusive backs wil get brought down alot easier, but if they have 10 runs and the first 9 are for 20 yards total and then they break off a 30 yard run for a TD thats still a 5.0 yard avg. You are not always going to get 4 or 5 yards on every carry especially against good defenses.
I would also look at your play calling. If you are routinely calling the same couple of runs on first down The defense will react and shut it down. You have to be willing to run at all times and to try and catch the defense off guard.
Again check out the slider forum and after any fumble go back and look at the replay and seriously think about challenging the play. One of my biggest gripes in this game is the fact that I see at least one overturned fumble in every single game. It gets kind of annoying.
Yes you can switch teams at the end of the year when your owner evaluates your performance, but be careful and save before you do. Because if you try and switch teams and no one is hiring its game over and there is no guarantee that the team you want to switch to will want a new coach. Although the Raiders seem to switch coaches almost as often as they do in real life.
Unfortunately you cannot change playbooks mid career. You can steal plays from other teams at the end of each game, but you cant just overhaul you entire playbook. If you want to use a custom playbook you will have to go back and restart your career with that set as your playbook.
If you have any more questions or need some clarification just come back and ask and welcome to on of the best football games ever made and be prepared to get addicted.