(As an aside, I have always hated how characters get free actions but can walk for "free", but this isn't considered a "free action." That's needlessly confusing.)
You can run for "free" by taking a free action

(while still using your two simple actions or complex action for other things).
if you run you can't for example use multiple attack to attack more than one target or call a shot.
you get -2 dice on ranged attacks if you are running at the same time
...but if you are using melee(!) you get +2 dice on melee attacks if you are running at the same time (running is great if you use melee!)
ranged attackers that are in melee combat have -3 to attack with ranged weapon.
(if you run up to an opponent and attack him in melee he will have -3 dice to attack your friends and since you are running he will have -5 dice to attack you)
defenders in melee targeted by ranged attack give -3 dice to defend against the ranged attack.
(if you run up to an opponent and attack him in melee he will have -3 dice to defend when your friends shoot him).
Defenders (both against ranged and melee attackers) get +2 dice to avoid getting hit when running.
(you run to charge attack someone, everyone have -2 dice to hit you with ranged or melee weapons).
Defenders get +4 dice to avoid getting hit when they are sprinting (this modifier is hidden away in text on top of p. 163)
(if you declare that you sprint on your action phase you can't take any other actions until it is your time to act again or the end of the combat turn, during this time you are considered sprinting and get +4 dice to avoid getting hit from ranged and melee weapons).
And then you have Interception...
If your target decide to move out of melee range (to avoid all them negative dice pool modifiers) the melee guy get an option to attack him with an interrupt action (this let you attack the same target more than once in the same action phase - once when you charged him and once when he tries to move away to have a chance to shoot without being considered in melee).
The interception interrupt action cost 5 initiative score (but it will not cost your action phase if you have not acted yet this initiative pass - unless of course you run out of initiative score before it is your turn to act). You can also use this interrupt action to take a swing on anyone that just tries to move pass you on their action phase.
The only way a target can deny you an interception if they spend their full attention on it (it cost a complex action) by taking a Agility + Gymnastics [Physical] test
(they spend their full action phase doing nothing than avoiding interception and moving)
Another benefit for melee is the way movement is resolved in SR5
consider 5 agility.
the first action phase you can choose to move right into the middle of the opponent gang of 3 people (up to 10m away)
use your free action on multiple attack.
swing your weapon once on everyone in range (up to weapon skill / 2 (round up) targets; with weapon skill 5 you can attack all 3 targets)
doesn't really matter if you hit or not...
- the point is that all targets you attack are now considered being in melee.
This mean all 3 targets now get -3 to:
defend against your friends ranged attacks.
attack with ranged attacks since they are considered to be in melee.
...And if anyone of them leave you can spend 5 initiative score to hit them with a free melee attack
(even if you already acted or are going to act in the initiative pass)
in the following action phases you still have little less than 10-20m left you can utilize for movement (depending on how much you moved in the initial attack). If there is only one target left or if all targets are scattered so you can't lock more than one in combat anyway then you should make a point in moving enough to be considered running so you get harder to hit and so you get a positive dice pool modifier to hit your target.
Another option would be to burn your free action on running and move slightly just over 10 meters (or all the way up to 20(!)m if you like) in your first action phase and be considered running for the entire combat turn and charge attack a single target.
you get +2 to attack your target.
your target get -3 to defend against your friends ranged attacks.
everyone get -2 to hit you.
you get +2 to avoid getting hit.
your target is locked at that position, if he move you can attack him again for 5 initiative score.
your target get -3 to ranged attacks (so total of -5 if he want to hit you with his ranged weapon).
in the following action phases you still have 0-10m you can utilize for movement (depending on how much you burned in the previous action phase).
even if you don't decide to move anymore you will still be considered running (+2 to attack, +2 to defend and everyone have -2 to attack you).
If there are no targets within range you can spend an action to Take Cover which will give you another +2 to +4 dice when defending (depending on how good cover there is available) that will stack on the -2 they already have to hit you. Another option would be to spend your action phase on taking a sprint test since that will give everyone -4 instead of -2 dice to hit you and it will also give you even more movement to utilize during the action phase... or simply pull out a weapon and shoot something (but you will have -2 to hit since your are still considered running).