Negative split run/walk calculator?

Bit of a niche question here. There are lots of negative split calculators on the web. There are lots of run/walk calculators on the web. But I haven’t been able to find a negative split run/walk calculator. anyone know of one?

Context: long time runner, running my second marathon (late to that game). Always run negative splits. But my legs aren’t feeling great these days and am dealing with some light injuries so I’m going to run/walk this one and take care of myself.

Thanks!