Using the D key simply comes from comfort using the WASD key standard. I opted to go without auto-run originally because the intent was to give the player the option to slow down if things got too intense. I could add another key as an auto-run toggle, or make it a menu option.
There are a few obstacles that have slightly longer or shorter hitboxes, but I need a clear way to communicate that visually and tighten up the hitboxes on the player to make sure there are no "invisible hits" when the character stands or lands.