I've just done that.

I decided to use one suitably housed 12V transformer ( a proper iron
one rather than an electronic one ) and run four sets of 12 V lights
from it - each set drawing a total of 20W. There's enough capacity to
run more lighting too.

The cable I used was quite thick speaker cable. A surplus shop was
selling 100 ft reels of it for a few pounds. As it turns out, apart
from the colour, it's identical to what was supplied with the lights -
it works with the insulation displacement connections that they employ.

I buried long lengths of corrugated round plastic 20mm conduit to
protect the cable. Then drew a cord into them by using a vacuum cleaner
to suck a small wad of bog paper tied to the cord which was fed in from
the other end and then the cables were pulled through ( leaving another
cord inside in case another cable needs to be added later )

Each conduit carried two or three cables, one for each set of lights,
so that voltage loss was minimised. I used commercial waterproof
connectors mounted in small plastic IP66 boxes so that the lights can
be disconnected if needs be.

The end result is great and my partner is delighted with it.