As long as your ratio was correct and you mixed thoroughly, I'd have to agree that the reason it is still soft is due to curing temperature. The stuff inside the crack will be just as soft as what squeezed out. I wouldn't get the area too warm because most epoxies begin to break down and soften at around 160 degrees. Keep your clamps or rubber tubing in place until it is fully cured.
And next time, consider using Titebond II wood glue for repairing cracks in non-oily wood. In cracks and other close fitting joints, it is stronger than the wood, and it makes a less visible joint in walnut than epoxy. Epoxy may be a better choice for cracks that can't be clamped completely closed or where larger gaps need filled. However, bubba repairs like that have no place on nice guns.
