Running head on once you see an enemy is certainly one way to do things, but for my money you should instead be predicting where they'll go next. Map layouts are pretty straightforward so you won't struggle too much to memorise their general shape, and if you see an enemy run in one direction, you'll know which direction to head in yourself in order to head them off. Stick behind cover and don't run about too obviously, and you'll likely get the jump on your enemies. Well, so you hope.