You can always start small and over decades grow the area. After all that is how cities like Copenhagen and Amsterdam became bike friendly, not just a few years, but decades of work.
As a fellow European: we're prone to underestimating how uninhabitable bits of America are that nonetheless have people living in them. Those are port cities and therefore stable and temperate. You cannot green Arizona.
Just FYI there's a lot of ways to re-green a desert without actually being wasteful with water. There's some really impressive case studies out there. Shaping the land with berms and swales, building walls of trees to prevent water from being leached away by the wind, etc.