Distance or Difference simply means "What Separates us or what Stands between us",
It's just a different way of saying: "If He decided to appear so close to us from the Start, why don't We just Attack him directly",
If Mihawk was testing WB's Legendary Strength, He would Use a much more Powerful Attack,
But He knows that as Leader of Invasion, He cannot be allowed to be the First to Test any Attack,
So Mihawk wanted to check "Who is gonna delay us from just Engaging WB directly?",
And the Answer was Introduction of WB Commanders
The Same Phrase can be used if for example you met Celestial Dragon Charlos but don't know who are CDs,
You'd wonder what Stops anyone from just Attacking them directly? The Answer is because Admirals, Marines & WG stand in between,
It has nothing to do with Strength nor Distance
Zoro also does this Test multiple times, He did it against Hody, He did it against Hawkins, He did it against Orochi,
Why these Three? Because They weren't Alone nor Close, so you need to first check with a Small Ranged Attack,