Quite happy that I was able to finish the hike on day 20 of a juice fast! :)
Half Dome is about 16 miles (26 km) round trip with 4800 ft (1 mile; 1.6 km) elevation gain. In the end, we have to climb about 400 ft (122 m) on an exposed rock using cables and poles dug into the rock.
In the first 15 days of the juice fast, I hadn't hiked at all. However, I used to go to the gym twice a week. On Day 16, I organized a 7-mile Four River Crossing Hike in Henry Cowell Redwoods. At the beginning of the hike, I wasn't sure if I could actually cover this distance on a juice fast. As the hike progressed, I felt totally normal! At the end of the river crossings hike, I felt that I could actually go much longer and the idea of visiting Half Dome occurred to me.
Having gone to Half Dome several times in the past, I had a good idea of various sections in terms of difficulty and exposure to sun. Still, it was challenging for two reasons: (a) the ongoing juice fast, and (b) my starting backpack weight was 21 lbs (9.5 kgs) with almost 15 lbs (6.8 kgs) of fluids.
A mental technique helped me a lot. I started at 6:30am. Quickly, I realized that I was slower than normal. I thought, "at this pace, I can't finish this hike before sunset". So I said to myself, "let me reach Nevada Falls (3.8 miles one-way) at a relaxed pace. Then I'll decide whether to turn back / continue". This became my mantra for the entire hike. I always focused on the next landmark (Vernal Falls, Nevada Falls, Ranger Station, split for Half Dome / Clouds Rest, location where ranger checks Half Dome permit, start of cables, top of Half Dome). Mentally, I was prepared to turn back whenever I felt like. That was very relaxing - no pressure to reach the summit :) I kept going, reaching one landmark after another. Even on the way down, I focused on the task at hand - to reach the next landmark.
Whenever I felt tired, I took a break for stretches and juice. After climbing the cables, I got an inkling of cramps, so I mixed some electrolytes in water. Luckily, I never got any cramps. I did get 'weak foot arch' issues - that slowed me down.
The toughest portions were (a) the first 1.2 miles (heavier than usual backpack, first 30 mins are usually tough as the body starts getting used to the idea of hiking), (b) The last 1.5 miles near the summit, including the cables (exposed; sunny; steep; i had to focus), and (c) the last 1.5 miles (I was tired).
On the whole, it took me 12.5 hours round-trip (about 4 hours longer than it has taken me in the past), followed by 4.5 hours of driving back home.