My girlfriend and I backpacked this trail in September 2015. we did two day to Johnson lake. got a late start so we didn't make it to Royal Arch. it is possible to make it to Johnson I. on day. it's about 14 mils. Good fishing and there was still water in most of the streams.
If done in the full distance, as I did, it will be rather strenuous. Lots of inclined switchbacks with little to no river view for about 2/4 of the time. The long haul is worth the climb though. there is about 3 pools to stop at and the tallest has a plethora of holes to swim in. CAUTION!!! These pools that move slower than the rest of the river do have leeches in them. Check your body before you decide to go back down. bring water and walking sticks will help a ton.