I have been working to break a bad habit I have, wherein combats are almost always resolved the same way, until one or the other group is dead. It is not really the way battles are fought, even between small groups of people, except of course in the movies.
You may take any number of battle scenes: Henry V, Braveheart, Spartacus, Gladiator, the Lord of the Rings Trilogy ... and the onscreen depiction is classic: side A and side B stand facing one another, until one force rushes into the other, or both forces rush together. What follows is a mash-up of slaughter and butchery, with no one in close order, until virtually all of the enemy is dead or fled. My D&D combats have tended to follow this motif ... mixed up as much as possible, particularly in how the combat is joined, but still with the central theme being that side A and side B duels to the death.
The fault, I feel, begins with the rule that states the participant that stays is entitled to one free hit on the participant who takes flight. This tends to reduce a party member's willingness to turn and run, and my willingness to give parties a free hit on NPC's ... the result being that battles, once joined, tend to stay joined.
Recently I have taken note that fighting between participants in actual combat do not tend to stay in close proximity to one another; six to eight feet is quite consistent, with participants moving forward to smash and blow a few times before breaking off again to judge one another.
Now, there's some argument in that for rounds being longer than they are (I've recently expanded a round in my world to 12 seconds from six) but I don't want to talk about that here. I merely make the point to say that, if two persons were eight feet apart, and one chose to run, the other could hardly have a convenient last moment stab. Consider the distance between Inigo and Count Rugen, just prior to Count Rugen's turning and running.
I also point out that accounts of battle that we have show that participants are quite able to pull back from the heat of melee as a unit, so that most large scale battles are fought by driving off the enemy, who reforms for another attack, falling back again and reforming again, giving up ground and retreating to a better position, the tide of battle turning back and forth with both sides remaining in close order and not flying apart into disarray. In fact, disarray is a sure and certain way for your force to get itself wiped out ... it is, in war, the WORST form of fighting. Two battle sides who were both in disarray would be blown to retreat, in the hopes of reforming into ranks before the other side was able, and therefore crushing the opponent.
In D&D, where the skirmishing is usually small scale, a combat between a very tough band of characters might be broken down more by small attacks by groups of 5-8 goblins, rushing in from different points of the compass, hoping to get the best opportunities from initiative, throwing a few spears, exchanging a few blows and immediately running away. Meanwhile, bowmen continue to harrass the party. Given the party tendencies for heavy armor, it would reduce their opportunities to chase down these goblin groups. Before, the party could have killed three goblins every time they turned to run away ... but no longer.
Most of all, this does not happen in the space of twenty minutes, but in the space of twenty hours ... said goblin harrassers jumping forth just long enough to force the party to keep awake, themselves sleeping and resting in shifts, while the party gets more, and more tired. Until the party is feeling the characteristics of forced march.
So that a high-level party which could usually handle 60 goblins in a stand-up fight finds themselves wittled down bit by bit, getting more and more freaked out as the damn, bloody goblins won't ... leave ... them ... alone!
Their only hope is to find a defendable place, hopefully with a source of water - or even better, the sight of civilization, before it's too late!