Interconnect Length

What kind of cart. are you going to try to use? With a MM cart. the capacitance of 20' will probably kill you. With a MC I'm guessing that only a balanced connection has a chance of working out.

In a phono cable 3 ft is best for sound.

Are you connecting the phono cartridge directly to the phono stage of the pre-amp, or are you connecting to the output of a seperate phone stage?

If the former I agree with the comment about 3ft. If the latter you could stretch it out a bit, but still wouldn't strech it out to 20ft. I keep all of my IC's no longer than 10 ft and use 3 ft when possible.

THanks for helping - I don't even know what cartdrige I would use - I have not touched nor consider getting a turntable since I am 14 years old... I am out of the loop. 
All I know is that I would use RCAs into my integrated, and I would place the turntable on a dedicated stand, to avoid all vibrations from walking and playing the sub on hardwood floor.
Maybe I could place it on the left corner of my room... (see my room in virtual systems).

Is it possible to mount your TV on the wall? That way, turntable would fit on the component stand.

Low capacitance cables should be okay! 
https://www.bluejeanscable.com/store/audio/index.htm

Don't put your turntable in a corner -- this is a bass node and will maximize interference with your table. Ideally as others have noted the cable to the phono stage should be as short as possible.

One option to consider is getting a small phono stage that you locate next to the table connected by a 1M cable - then run a longer cable from the phono stage to your amp. Of course your amp has a MM phono stage built in so you could go with a head amp and a low output moving coil and the run the longer cable to the Marantz. Even a step up can help -- I currently run 2M cables to my step up then another 2M to my phono stage and have no issues

@goheelz  yes ! - that's what I was thinking.  I don't know how to elevate the tv; I Know I do not want to mount on the wall.  
Maybe I can place it under a stack of hockey pucks right under the "feet"... All I need is 5inch of clearance.. I just have to look for a base, maybe build one.  The TV is 6 years old, so it is heavy by today's standards.

either move stuff around, or...

get a good separate pre-pre and use XLR balanced runs

chrisr, how high do you have to 'elevate' the TV?  'Stacking hockey pucks' might work, but my initial response is 'stability issues'....

Don't know if you've pet(s), children (of all ages...;)....), or just the 'random event' that could trigger the 'house of cards' event....*G*

...just 5"?  Or...?

Another possible option (if your parents will let you) is to mount a TT shelf to the studs of the rear wallor some type of combination of the above ideas.

Take folkfreak's advice, it's good.

Dear @chrisr : No more than 2m., it does not matters balanced or unbalanced IC cable. The cartridge very low level and delicated signal as more length it must run in a cable as more degradation suffers.

Regards and enjoy the MUSIC NOT DISTORTIONS,
R.

Balanced cables are all I would ever use for anything over maybe 2 meters or so…and that's a stretch. It seems like balanced phono cables are a great idea as phono cables can use all the help they can get. My old Linn had a crappy tonearm cable that was improved amazingly when I bought a Jelco/Mogami cable…no hum from the cable picking up dimmer crap, and simply cleaner overall sound.

Here is the only way to run long runs of line level audio; you have to use a sending and receiving "Balun" plus Cat5e/6 cable


              http://muxlab.com/product/stereo-audio-balun

Buy the baluns, and make your own RCA cable for higher quality. The Cat5e/6 cable can be bought in various lengths. There will be no degradation.


The connection will be; RCA to balun (a few inches) 20 feet Cat 5e/6 to receiving balun, RCA (short) to line in.

     
     

No more than 2m., it does not matters balanced or unbalanced IC cable. The cartridge very low level and delicated signal as more length it must run in a cable as more degradation suffers.

You do want to keep the cable short, but not for the reasons stated here.

The problem is capacitance. The cartridge has an inductance and the cable is a capacitance. Together they have a resonant frequency. Normally its in the RF region or ultrasonic. But with 20 feet it could well be in the audio range. The peak caused by this resonance is often over 30db!!

If you really need to run the cable that far, consider a phono preamp with the turntable, and then run a line signal the distance.