I've done the solo one-day. I'm a petite female (under 5 feet tall) and started at 1 am. I never felt afraid or alone... lots of friendly folks on the trail passing me by. (I later caught up to most of them on the switchbacks... I'm the tortoise, slow and steady.)
You may be lonelier if you opt for the Old Trail, but I'd advise it to avoid the North Fork creek crossing in the dark that time of year.
