Even in the my prime of my youth I don't think I could day hike South Lake to Dusy Basin and back again, all in the same day. Must be near 18+ miles rt.
It's only 16 miles and little elevation gain, 2200 in and about 600 out, according to my GPS. It's not difficult at all, not nearly as tedious as day hiking Whitney. But Dusy Basin (to me) is more scenic.
I love Piute Pass too into Humphreys Basin. Did that yesterday and saw only 2 people the whole hike.