I haven't gone there and back in a day, no, only camped there. But on different trips I've hiked to Ediza, hiked to Thousand Island on the High Trail, taken the JMT between Garnet and Thousand Island, and hiked almost the entire River Trail in two different chunks. I'd say that if you could do the Ediza 15-mile round trip, you could do this 18-mile one. There are long stretches of the River Trail that are quite easy and cruisey.