Since I have a spreadsheet open (taking a break from work), I'll put Karls' comments in perspective. The electronic signal would arrive at the sub-woofer about 2 one hundred millionths of a second later than your mains for an added 20 foot run. Much less time than mere humans can notice. However, a twenty foot difference in distance means about 2 hundredths of a second lag acoustically -- i.e., from the speakers to your ears at the speed of sound in air. That may be noticeable -- ears are pretty keen regarding distance and direction.
If it is noticeable to you, keeping the distance difference to a few feet could help. A bigger concern might be directionality. Depending on how you set the cross-over on your subs, location will not be directional. That should help in the set-up. If the sub reproduces sound above 100 - 140 Hz, you also need to worry about directionality and that means you need to place the subs near the mains. Otherwise, you only need to (as Karls recommends) worry about the relative lengths of the signal paths to the listening position. Good luck.
If it is noticeable to you, keeping the distance difference to a few feet could help. A bigger concern might be directionality. Depending on how you set the cross-over on your subs, location will not be directional. That should help in the set-up. If the sub reproduces sound above 100 - 140 Hz, you also need to worry about directionality and that means you need to place the subs near the mains. Otherwise, you only need to (as Karls recommends) worry about the relative lengths of the signal paths to the listening position. Good luck.