Since the speed of the game has increased so substantially, many shots hit by your opponent seem impossible for you to run down and return. A great way to work on covering more of these seemingly unreturnable shots is to get in the frame of mind that you are going after every ball regardless of where and how it is hit.
Keep your focus on the ball and your opponent's racquet. Once you see the direction that the ball is traveling, turn your shoulders and go. Even if the ball looks like it is going out, or if it looks like you have no chance to get there, move in that direction right away. In practice, try hitting all of the out balls back and hit balls out of the air or on the short hop (half-volley) if you cannot get in optimum position.
I use a drill (like playground tennis where everything is in) where all players on the court (usually two or four) have to hit the ball back if it lands anywhere inside the fence. This takes away the initial determination we tend to make on whether the shot is going to be in, whether we can get to it, or how well we are going to be able to hit it. By eliminating this thought, it makes you react immediately, increases your speed to the ball, and gives you a great chance to get your racquet on almost every ball. It is not all about the speed that you can run-a lot of your court coverage ability will be determined by how quickly you react, how soon you start to move, and how decisive you are about running after every ball.
Try using the playground tennis idea in a match and if the ball is going to land out, run towards the ball and just let it go. I am confident that you will notice a big difference in your ability to run down your opponent's shots, and also get a lot of those seemingly unreturnable shots back in play.