I'll tell you how it works in my campaign world, and then you can decide how you go from there:
The elves don't consider anything that sheds blood to be a particularly honorable profession. Many in fact consider it down right dishonorable. So they don't have much in the way of a standing army. Normally, threats are dealt with locally by an impromptu organizing of the militia. Elves are very xenophobic (plagues hit them hard), and they practice the 'shoot first' philosophy whenever they have the advantage in numbers on the principle that anything wandering in elf lands that isn't an elf is probably an enemy or too stupid to reason with. Elves that avoid combat live for a long time, and they typically obtain a fairly high level compared to shorter live races. As such, in most places the 'border patrol' may consist of a random elf who happens to be near the border when he sees 'invaders'.
Average Elf (Com8): CR: 2; Medium-Size Humanoid (Elf); HD 8d4-8; hp 12; Init +1 (Dex); Spd 30 ft.; AC 12 (+1 Dex, +1 padded); Atk: Longsword +4 melee (1d8), or shortbow +5 ranged (1d6); SQ: Elven traits; AL: CG; SV: Fort +1, Ref +3, Will +3; Str 10, Dex 13, Con 8, Int 11, Wis 13, Cha 9
Skills and Feats: Animal Empathy +2, Craft +10, Hide +3, Listen +7, Move Silently +3, Perform +4, Search +4, Spot +7; Alertness, Skill Focus (Craft), Run
In areas that the elves haven't tamed, your average elf is a bit wilder and more martial in bearing - in no small part because these less civilized elves rely on hunting for their food.
Average Nomadic (or 'Wild') Elf (Rgr3/Brb2): CR: 4; Medium-Size Humanoid (Elf); HD 3d8+2d12-5; hp 22; Init +2 (Dex); Spd 40 ft.; AC 16 (+2 Dex, +3 padded, +1 buckler); Atk: Longsword +6 melee (1d8+1), or longbow +7 ranged (1d8); SA: Favored Enemy (Animals), Rage; SQ: Elven traits, Uncanny Dodge; AL: CG; SV: Fort +4, Ref +7, Will +2; Str 12, Dex 14, Con 8, Int 9, Wis 10, Cha 9; Skills and Feats: Hide +7, Listen +7, Move Silently +10, Perform +4, Search +6, Spot +7, Survival (Forest) +5; Endurance, Rapid Shot, Run, Skill Focus (Move Silently)
Note that the CR is by rule of thumb.
The elf will rely on his superior senses to detect the foe before he is detected, and flee quickly through the woods. Facing invasion, the elf will probably try to get about 30 of his friends and ambush the danger at a distance from cover or concealment. Since the above elf is probably living out his 4th century, he's in no hurry to risk direct contact with a dangerous foe - elves that do seldom live so long. Generally speaking, a hunting party of 30 or so wild elves is enough to strike fear in most anything. You can imagine how they might seem to a column of 1st level warriors stumbling about in the woods. Even a veteren army doesn't enter elf woods lightly.
In most places where there are large established elven kingdoms, they've arranged for border defences. These consist of magical traps of various sorts called collectively 'the Wards'. They are usually tied to trees in the forest, and they typically do things like create permenent magical fogs, mess with tresspassers sense of direction so that they become lost, cause tresspassers to become confused (and attack each other), or mislead tresspassers with flitting illusions that always flee away from them. Some of the inner ones are more akin to 'Glyphs of Warding' and can do actual damage to the unwary. These are usually set along false paths. The Wards have been slowly built up over thousands of years so that there are literally hundreds of thousands of them forming a dense magical fence around their kingdoms.
However, the Wards require maintenance. Foes of the elves cut down the trees to disarm the wards. Less malicious but foolhardy sorts cut elf wood for lumber. Old trees die. Discharged wards have to be renewed. The job of policing the borders and maintaining the wards falls to the Wardens. Because the Wardens are basically soldiers, they tend to be home to social outcasts, pariahs and misfits that just don't fit in with the other elves. These include misfits by alignment disposition (everything from LN to CE), misfits by social ability (low charisma), the rare half-breed, and non-xenophobic elves that associate with other races. Indeed, the rare elf-friend of another race may be invited into the Wardens.
The Wardens are essentially the elven warrior and adventuring caste. They are a relatively small elite group, with an average level around 6th, well equipped, and usually elite stat arrays. Leaders are generally not higher than 8th level PC classed, and noviates are generally not less than 4th level. For an array of 10,000 square miles, there might however only be 40 or so of them, moving in teams of 10 and actually constituting the bulk of the 'elvish army'. Each group will contain at least one and probably 2 arcane spellcasters, and at least one and possibly 2 divine spellcasters. Essentially then, each team is a fairly large and rather capable adventuring party.
To patrol such a large area, they rely extensively on their natural connection to the world, using animals and nature spirits as their eyes and ears and collecting gossip from them as they move about. They always attack from ambush if possible, from range, preferably in the twilight if their foe is impared at night or in broad daylight if they are impared by the sun, preferably while the invader is bewildered by Wards, and they almost always employ magic. They generally camp in the wild, but they will have prepared a number of well concealed safe places that they can fall back to in the event that they are overran or injured and need to recuperate. If need be, they'll hide and then fall on the enemy rear, killing stragglers, destroying supplies, and otherwise harrassing and making themselves a deadly nuisance. Elves just don't ever stand up and fight. The old saying, which is half-true, goes that an elves always prefer to fight their battles on another day... a hundred years from now.
