Install another (presumably) 300W xfmr, run new cable to mid point of LV light run, splice in there, so that the 1st xfmr has half the lights (or distance), and the 2nd has the other half.
or, from the last light, run a new cable back to the xfmr, and cut the circuit in the middle. you may need to upsize your cables from the 1st light to the transformer.
how many and what wattage lights did you install, what size cable did you install, and how long is the total run of cable?