It’s easier to make the character run in place while the planet rotates under him unless you plan on doing a ton of C++ coding. UE4 usually only recognizes gravity as pointing downwards and it takes some fancy footwork to get a character running upside down. So what you want to do is figure out how far the planet rotates and/or turns based upon the character’s speed and direction - then generate random level assets (which are attached to the sphere) just before they come into view over the horizon.