You''ll find all types of mission starts. Some of the player-made campaigns have an annoying habit of dropping you off right smack dab in the middle of the the enemy formation before you've even had time to open up the map...

...just between you and me, I hate those types of missions.
Other missions start you off a hundred miles from the enemy that you have to intercept. The mission descriptions give you a vague idea of where they problably are and going and you may not even be able to detect them yet. Its up to you to plot the intercept course and speed to put yourself into strike position or, if your already in the right spot, hide quietly waiting for them to come to you before you pounce.
Other missions have various starting points in between those two extremes. There are about 70+ player-made single player missions (on SubGuru's website shown below) that have been made by the community to complement the stock mission and 4 extra campaigns (though a couple have bugs), so you'll find all types.
http://www.subguru.com/dwmissions.htm
hope that helps
lb
EDIT: Wrongwebsite. Corrected above