The Thunderbolt 2 adaptor has a USB-c connector to attach to your mini, but there is no USB protocol involved in that connection. Everything is Thunderbolt and the only way to extend the length is to add Thunderbolt cables to the mix.
You could get a longer TB2 cable to go from the adaptor to the screen, or you could get a long TB3 cable (in theory, I don’t know if they exist as extension-style male/female cables) to go from your mini to the TB3->TB2 adaptor. With a copper cable, the maximum length for an active cable is 3 meters (just about 10 feet). To go longer (up to 60m or about 200ft) you would need a fiber optic cable, and those are much more expensive. As in hundreds of dollars for a 10m cable, and over $1000 for a 60m one.
Edit: if you have a powered dock that does TB passthrough (either 2 or 3) then the maximum cable length is reset at that device. So, if you have a 2m TB3 cable to your TB3 dock and then plug your TB3->TB2 adaptor into it, you can hang a 3m cable there to go to you display. You might be better off, money wise, buying a new display.