Very cool and potentially deadly, but with the drawback of the duration.
11 rounds to become exhausted, so 6d6 nonlethal damage for a 17th-level caster. True, the target can't do anything while running but it's simpler to just use finger of death. I'd reduce the spell level.
Reduced to 7th because I feel that exhaustion and fatigue are pretty powerful... actually make that 6th b/c immunity is common
Homebrew: If it is mine feel free to PEACH and/or use it.