I'd get a decent quality shielded cable, like this:
https://www.parts-express.com/35mm-stereo-male-to-male-super-shielded-audio-cable-50-ft--240-077
Can you cut and splice and have soldering skills? If so, remove the mini plug from the other end of the cable (you don't need it), strip the jacket, keeping the shield that covers the red and white conductors, pull and twist it into a bare, stranded wire. This is negative. The red and white are for R/L, but you need a mono signal. Check your sub's manual to see if the line out is mono; it should be. Then you can strip and twist the red and white wires together. This will reduce the wire's resistance for the long run. For the RCA end, if you have a plug like this:
https://www.parts-express.com/gold-rca-super-plug-connector-black-63-mm-cable-entry--091-052 , then you can connect your cable directly to the plug, which is best. You'll have to solder the signal (red/white) to the center pin. If you have a single RCA cable with plug (which can be cut off a cable you aren't using), strip and prep for soldering. Before you twist the wires together (same as long cable), slip a small piece of heat shrink tubing over the R+W connection area to be soldered and add a larger piece, which will fit over the outer jacket of the cable, long enough to cover the splice and extend over the jacket at least an inch or so. . Be sure you have enough room to solder the signal wires without shrinking the tubing! Then, use a heat source (lighter) to shrink the tubing. If you prefer to use tape, be sure to completely cover the signal wires with a few wraps, pulling it snugly as you go. To wrap the outside, wrap the tape starting on the jacket, over the taped connection and to the other side. Be generous and pull it tight for a nice durable job.
Here's a link to heat shrink tubing. Choose the right size, or select an assortment. It's good stuff to keep handy.
https
/www.parts-express.com/Search.aspx?keyword=heat shrink tubing&sitesearch=true