You could start with this in your character bp (vector curve x,y,z at 0 time 1.0 and after 4 seconds 0.1 - and speed curve 600 at 0 time and 60 at 4 seconds):
However you would have to adapt your anim-bp to “scale” the speed value as well - e.g. speed / scale (or the blendspace would switch back to “walk”). If you don’t decrease the max walking speed it would not look that great (as the character would be way too fast if he is small like an ant). Physical assets (ponytail, ragdoll) could make issues if you scaled down. So probably you have to switch it while scaled down or there is a possibility to fix that issues (did not try). Other than that it looks fun 