All you really need to do is to create one huge burrow that covers the entire map, and then order your dwarves to defend it. They will run around the map to stab at anything that turns hostile regardless of where it is on the map.
I recommend having some sort of guard post at the surface though, but you can do this by making a smaller burrow, such as just in the gatehouse itself, and have another squad of dwarves defend the gatehouse.
This means anything trying to sneak into your fortress will encounter the soldiers in the gatehouse/barracks. You will then have reinforcements come running, eager to stab more things. Once its all set up the only thing you need to do is replace any "vacancies" in the squads. If you rotate the squad schedules you can even have your guys doing training and sparring, preferably in the gatehouse/barracks so there are always soldiers there to catch anything trying to sneak in.
For example, lets say I want a large military, yet still have each squad small for efficient training, do this:
12 squads, 3 dwarves each
Each squad gets 2 months on training in the gatehouse/barracks while still on active duty.
The other 10 months have them defend the burrow that encompasses the entire embark.
At any given time there will be 6 dwarves in full gear as soldiers in the gatehouse.
There will be 30 additional dwarves roaming around the map to stab at anything hostile to your civilization, including hostile wild animals, megabeasts, and Urist McNeedsashell.
Your fortress defense will then handle itself, and you only need to worry about replacing the occasional "vacancy" in your military roster.