I don't recommend this hike if you're not fit or too old. Getting to Hanakāpī‘ai Beach via the Kalalau Trail was really hard for us. It takes a lot of energy, and more difficult if the weather is hot. The trail was very rocky and muddy, and a significant amount of steep climbing required. It took us 2.5 hours to get to the beach, and we gave up the idea of going to the Hanakāpī‘ai falls as we were too slow.
When we got back to the trail head, our legs were stiff from all the effort of the hike. We had to sit for 1/2 hour to recover our energy and legs.
If you're just hiking to Hanakāpī‘ai Beach and not to the falls, I honestly wouldn't recommend it given the effort you need to get to this beach. Save your energy for the other beautiful beaches in Kauai!