Does the length of the Cable matter?

Unless I’m mistaken, the input impedance of your phono preamp is 47,000 Ohms. This means an increase in impedance from a long length makes no difference.

Shielding is the next thing. I have a 1.5m length phono cable. If that shielded phono cable crosses a power cord, I get a slight hum. Therefore make sure your long phono cable doesn’t get too close to a power cable.

The 15’ of cable would definitely be a mistake if your turnable is 16’ feet away from the phono stage.  Otherwise, you should be fine.

At 15 feet you may be entering balanced connection territory to eliminate hum and provide a stronger signal. Just my thought. I’d be most worried about hum. 

IF hum is a problem, adapters exist to convert from rca to Balanced, simply to use a better noise resistant balanced cable for the run.

convert TT preamp out RCA to Balanced, then at the amp convert back from balanced to RCA.

female and male versions exist

various options

As a general rule, as I have been told by repeated 'experts', keep the interconnects as short as possible, ~1 meter.  Amp to speakers can be much longer, but use high quality speaker cable, not lamp cord....