Can you recount the story from the Satcharita about the girl who was too weak to walk and how Udi helped her?

Certainly. A powerful example of Udi's glory is shared in a footnote within Chapter 33, as recorded by R. Hari Sitaram Dixit. The story concerns the daughter of Khimji Lalji Joshi, who was gravely ill and so weak that her limbs were like twigs and she could not walk. Her mother first obtained some of Maharaj's Udi, which provided some immediate relief. Subsequently, the family took the girl to Shirdi. As Chapter 33 recounts, Baba himself applied Udi to her and blessed her. In just a few days, she began to walk on her own, and her remaining difficulty was fully cured when Baba gave her a fruit to eat.