There are several ways of doing this.
You can pre-focus on an area (particular tree,rock etc) and set your camera either to A priority or S priority (depending on whether you want frozen images, or some with motion to them) and just before she enters the 'zone' start firing away.
If you have the sony flash you can place this in position to give an added dimension to the images and is great if light is poor.
Pan as she comes down the mountian/track and just keep firing away as she comes down, focussing on her body with an appeture of f/8 if light allows, then that should give you a good dof and sharp images.
Id either go with the 50 to get some close wide angle images or the 70-300.
The tamron 90 will be too slow focussing id imagine.
Let us know how you get on and show us some images.