qgis - How to create a buffer around a point that takes terrain elevation in account?

In QGIS, I am creating a few different buffer around points to simulate walking distances. For example, 10 minutes walk about 800m at 5km/h. Using population density, I then clip a population layer which gives me the population that is within a 10 min walk from those points. I am working on a hilly city where elevation does matter.

How can I adjust the buffer with elevation?