I think everyone is quiet because you have a rather challenging problem. I can't think of a clear solution, but there are a few things you may want to try:
1) Figure out if your wire is Copper or Aluminium (color would be the clear indicator). Copper is a 2x better conductor, so you will incur 1/2 the losses over nearly 150 meters (70m there, 70m back) of wire that you have to your zone 2.
Alas this will potentially be an expensive change, and it does not guarantee that the change will be sufficient for your needs, but if you are using an aluminum wire, you will definitely get a voltage increase by switching to copper.
2) Switch to a 10 or 12 awg wire and/or run several pairs in parallel. The thicker the wire and/or more wires you have supporting this far away zone, the higher the target voltage will end up being. For best results you may wish to explore using a copper power cable or an outdoor speaker wire.
3) (a bit experimental) You could get an AC transformer to "step-up" the voltage going to zone 2. Ideally you'd find a proper primary / secondary AC transformer with a "correct" turn ratio for what you need, but you could also try a standard AC transformer like this one (link), and connect black to common, white to Rachio's zone 2 output, and Red or Orange to the wire running to your zone 2 valve (make sure that valve is no longer directly connected to Rachio, but connects via the transformer). The transformers output (blue and yellow) should theoretically be left disconnected.
I warn you that this will work only on theoretical level, I've never tested this approach with Rachio / zone far away. This should nearly double your "down range" voltage, from 15V to somewhere under 30V.
There are more indirect methods too, such as an adjustable DC/AC supply controlled via an AC relay, but these would be more expensive so you may want to try other methods first.