Future Wastelands discussion

This is my first posting here so apologies if it's in the incorrect folder.

It was a thin-ish paperback, couple hundred pages. Dealt with humanity suddenly dying off with the exception of those people who were descended from people who had the Spanish Influenza (1919-20) who were somehow immune. It was found that these people were the "next" stage in human evolution. The main character is a girl who is travelling (but I can't remember where to or why).
I thought it was called The Walk or The Long Walk but it isn't the novel The Walk by Lee Goldberg or Stephen King's The Long Walk. I'm not even 100% sure it had "walk" in the title but that's what keeps coming back to me when I think about it.

I've gone through Amazon, B&N, Goodreads, all looking at lists and descriptions to no avail. I've also posted to the dystopianbooks Reddit group who offered a lot of good suggestions but also no joy.

Any bells being rung? Thanks in advance!

Do you remember anything else about the book? Anything about the characters, like their names? Or maybe a quote or something?

The main protagonist was a younger girl, early teens. She was very close to her grandfather. I believe the story is told from the viewpoint of their journey after the die off.

I read it perhaps 15-20 years ago so the memories are vague. But it also helps that it was published during a time when there were fewer dystopian novel out there.

I can't find anything. I've tried looking it up multiple times.

Thanks so much for trying, Bethany! I appreciate the effort!