Not sure it would be fun... but it's your hike.
It takes me about 3 hours to hike down SK with a 20+lb pack. I think 2 hours at a good clip would be a reasonable goal. Unless you're used to trail-running in the dark, I would think twice about running down before sunrise. (I just broke my ankle going down Tanner 2 weeks ago... trust me, it doesn't take much.)
It takes me 5-1/2 hours to hike out Bright Angel with a pack. My teenage son can do it in 5 easily. I think rangers regularly do it in 3... in sandals. In good shape, with a mission, 4 hours seems reasonable. I think the shade and water on BA makes it a much better choice than SK.
>> As for an emergency plan my wife will be with me camping but not on the hiking so having a 12 noon call the ranger time if havent returned to the rim. <<
FWIW, I think this is a borderline irresponsible emergency plan. If it's an unusually hot day, you should cancel your plans before you start. If you're not feeling great at the bottom, take a break and soak in BA creek. At Phantom Ranch, the Canteen has A/C. At Indian Garden, there are rangers and a full infirmary if you're not doing well. (They can even run an IV there.) Along with potable water, there are emergency phones at 1-1/2 and 3 mile resthouses.
You won't be alone on the corridor trails, even in August.