![[Yosemite]](/images/yosemiteicon.png)
Wilderness permits require trailhead of entry, trailhead of exit, and first night's campsite/destination. They are not at all flexible on where and when you enter the wilderness. Once you are beyond the first night, you can go anywhere you want on whatever schedule you want, exiting on the day at the trailhead listed.
You may find it too long to get all the way to the exit trailhead in 2-3 days, if you do the route I describe - there is some leeway here, as rangers will be understanding of people who overestimate their abilities - they're used to that. If you find you can't go up the Panorama trail and need to exit (downhill, to the valley) via the Happy Isles trailhead instead, they don't penalize you. The itinerary I suggested is one that I personally would take four or five days on, just because I love Yosemite and find myself stopping to soak up the scenery. But I am aware that there are people who can do 12-15 mile days without hesitation.
You say you're "in top shape" but you can't know what that elevation will do to slow you down. I am going into the mountains on average 2-3 times a month, as I live here in the shadow of the Sierras, and last week found myself huffing and puffing at 9,000 feet elevation - you don't always get a free pass for elevation. People join our hiking group all the time expecting an easy walk in the woods and then find out that driving to 5,000 feet and hiking to 7,900 feet all in one day overwhelms them. Oxygen levels are different up there. So, you may show up in the park, start hiking, and climb hills like they aren't there - or you may show up in the park, start climbing, and find yourself out of breath and struggling. Drinking plenty of water will help, as will frequent snacks/electrolytes, but in the end, everyone responds differently to elevation. Sometimes every single time. Most times, I am able to go without an issue. Sometimes I have problems. There's no way to predict it and no way to prevent it. Acclimation (staying at moderate elevation for a day or two so your body can adjust) can help. Going to the gym every single day of the week does nothing to prepare you for high elevation; it helps, but I've seen religious workout fanatics falter on the trail.
All that to preface this - the four mile trail uphill is typically regarded as a buttkicker. Adding 8 miles of Panorama Trail, then another six miles of trail to the summit of Half Dome, may or may not be possible for you. It would be easier for you to do Happy Isles to Half Dome. You might crash two miles into the four mile trail or make it to Nevada Falls and be thrashed, you might make it all the way up and then back out and wonder what I lectured you for. I don't know. You won't either, until you're there. But I can tell you that thousands of people underestimate the difficulty every year, and it's those people for whom the SAR filter water at the top of Nevada Falls day after day.
