80' of the monk.
40' regular movement + 40' dash.
And if they start further than 80' away, they won't reach anyone that turn. So just shoot a few arrows for free damage.
Okay, I mentioned the dashing as that takes your bonus action. Now why is the enemy wizard more than 30 ft away from any and all allies?
I don't expect to 1-shot a caster. Even with flurry.
But you can certainly force a lot of concentration checks.
And shadow monk can negate most of their spells that required line of sight.
I don't think "the shadow monk can..." is a good appeal to a base class identity.
And, you are only forcing two concentration checks, and only if the enemy caster HAS a concentration spell up. What if they don't? Then you have rushed back behind enemy lines to just attack a target that you don't expect to take out.
Then your fighter gets a free opportunity attack.
So, I want to make sure I'm getting the full picture.
The Monk's identity in combat comes from
A) There being a mage in the enemy group
B) That mage being a decent distance away
C) The enemy units being already engaged with your allies.
So... what do you do on turn one? Dodge and wait for the fighter who may or may not exist to engage with the enemies so you can run behind the front line to attack a caster that may or may not exist? Look, if you get your ideal scenario, it isn't a bad plan, but you are laying this thing out and assuming a lot of things are already in place.
Also, fireball is a 20' radius. So just the fact that you exsist keeps them in blast range.
No? They can hit you and not themselves pretty easily.
Want to assume an archer instead?
Or if they are all melee. Then you can just kite.
If you're just going to run at the first target you see and hit them... I recommend a Barbarian.
So run into a melee enemy, attack them, run out and get an opportunity attack? Or do you spend ki to disengage limiting the number of times you can do that, and the amount of damage you can do? Also, to kite, you need to be further than 30 ft away. Either the enemy has to oblige you by running towards you like robots, ignore you and focus on your allies, or you need a base of 65 ft of movement which you frankly cannot depend on getting.
Or, you decide to forgo using your monk abilities, and become a running archer.... which is something you could do as a rogue.
Also rogues can gank the backline too. A rogue who is in the same position as the monk you described, 40 ft from a caster or archer and 40 from the enemy melee, can use a light crossbow with Steady Aim and get their sneak attack, dealing as much or more damage, staying completely safe, and now getting additional options to harm and debilitate their foes.
So, your "monk identity" is a poor copy of the rogue's identity, with less support and more danger.