I'll second the recommendation for every 10'. Use a good sized wire, ideally 14ga. You can solder it directly to the power tabs under neath the fastrack, each piece has them.
I see your running DCS as well. You'll probably want to put your layout into blocks as well. I'd put a break in the center rail on each end of 10' of track with feeders at the center of each section.
You should also run those feeder wires back to an MTH terminal block fed by the TIU for the best DCS operation.
Legacy does not have a "preferred "wiring style. Wire your layout for DCS and add the Legacy command wire to the common input on the terminal block.
Heres a link to other DCS wiring topics for more info: DCS Wiring