Midi cable help

I could use some help and advice on a subject I know absolutely nothing about.  I have 2 keyboards both midi capable,  or so I believe.  One is a Korg SGPROX and the other is a K2500.  Both 20 year old keyboards.I'd like to explore the world of midi and I need cables that will work for these keyboards.  They will go into my Behringer Xr18.   Can someone tell me what cables I  need?  Also will the length of the cable affect performance?  I'm guessing in need 15 to 20 ft. cables.  The keyboards and mixer are on different sides of the room.As I said I know absolutely nothing about this feature so maybe what I don't know will keep me from doing this.Thanks, Don.....Sent from my Galaxy Tab® S2