The early section of the PCT is relatively low altitude starting at 2915 ft, rising quite gradually to a peak of about 9000ft until one reaches the High Sierra where altitudes exceed13000ft in one or two places. It is possible to take a detour to higher ground if certain mountain summits appeal to you, but the highest actual trail point is Forrester Pass at 13180ft. Inconveniently, this is about 29ft over the typical limit for most travel insurers, and one I called told me I'd have to insure myself for exceeding that altitude, over the duration of the whole trip, even though I'll probably be over the pass in half an hour and back under the limit.
The desert, is also the habitat of crotalus atrox, or the Western Diamondback Rattlesnake. Its not the only rattler though - there are many other varieties and most of them live in abundance here. You may walk a week without seeing or hearing one, but then see five in a hundred yards. They are potentially deadly poisonous, and can bite multiple times and lightening fast in a second or two. I say potentially as about 7,000 - 8,000 people are bitten per year in the USA, but fatalities number in the single digits. The good thing about them is they are not stupid, and when some lumbering hulk of a hiker comes barreling down the trail towards them, they know he is 50 times their size and could kill them, and immediately start warning of their presence with the rattling-tail gag. This is a good thing - no-one ever died from hearing a rattlesnake. so as long as you stay alert, don't step over something where you can't see the other side, poke your hand into bushes or under rocks and so on, you will be highly unlikely to get too close to one, and therefore never get bitten. Few rattlesnakes exceed 4-5ft, and can strike about half their body length, so if you walk six or seven feet round the snake, its not going to reach you even if it did try to strike.
Occasionally, a lazy or stubborn one might sit in the middle of a path,refusing to move. If lobbing rocks at it doesn't see it off, you may have go round it, if that's safe. Its pretty stupid to muck around with such a snake though, so best not to go poking it with a hiking pole or what have you. Most snake bites happen because people are interfering with the snake or trying to handle it - so the trick is, don't. If I met a snake in a confined gully, I would be only too happy to take a short reverse detour if possible, or settle down for a cuppa a few yards back until it buggered off. What I wouldn't do is piss it off.
Other hazards include scorpions, which of course have a stiong on their articulated tail - apparently, the ones with big claws are generally not very venonmous, whereas the ones with little feeble claws tend to be VERY venomous. Scorpions are nocturnal, so my plan is to keep my shoes and socks inside my tent when I remove them, and then I know there ain't any hitchhikers to shake out next morning. Even the most venomous scorpions rarely cause fatality to a fit person, but the sting is ghastly and painful from most of then, so best avoided.
The bigger animals to be aware of are bears, black bears, (which are not always black - sometimes brown). They are powerful, physically big animals, but seldon aggressive, and more likely to hurtle away at pace if you encounter one. A juvenile might be curious and follow a hiker hoping for food, but my feeling about bears is I will be pleased and excited to see one. It would be foolish to assume that there is no danger from bears though, and in some sections of the hike, especially the sierra there are places where bears have become familiar with humans and may come close, so close control of food storage is required to reduce their dependency on being near humans - this has become such a problem that bear-proof canisters are mandatory in places. In parts of Canada and certainly Alaska, the Grizzly bear (or what Native Americans called the "real bear") is common. These buggers are a whole different kettle of fish and I would not want to meet one of those much.
Then there are Mountains lions, also known as cougars, or puma. These big cats are common through the whole of North and South America. They are not as big as African Lions, being slightly smaller than a Jaguar or Panther, but still capable of killing a man. In fact there are rising numbers of attacks in the last two decades with some 20 deaths or so, which is twice as many as in the previous century. This is because of rising numbers of human visitors to their territories, although nearly two thirds of these fatalities were children. On the whole it is unlikely that you will be attacked by a mountain lions, as they usually don't recognise humans as prey, but a cornered or very hungry cat might be more aggressive. I really hope I get to see one, but they do say you are a hundred times more likely to be seen by one, than see one yourself.
I'm saving the worst till last obviously. Mosquitoes. Little bastards. Like tiny single engined aircraft whining around looking for someone to bite. In the summer in the Sierra, where water is everywhere, so are the mozzies, so its long pants, long sleeves, head net and bug repellent, and maybe you'll get away with out too many bites. But, you WILL get bitten and sometimes they are as thick in the air as smoke. I'd sooner meet a pack of wolves.
Presuming one is not inconvenienced by any of these creatures, the walking will be tough, depending on how fast or slow you take the journey. The thing is, you can't be too slow because of the weather.
Most people start in mid to late April, and indeed there is a big 2 day kick-off party about 20 miles up the trail at the end of April. Start any sooner than this and you risk getting to the Sierra Nevada while there's still too much Nevada going on - snow. And it can be many feet of it. Too late, and the desert will be getting hot and dry, the melt water in the creeks in the Sierra will be raging torrents, and you may not get top the end of the trail before the snow starts falling for next year in Washington. The journey takes most comleters about 5 months. My plan is to do it in four and a half, starting from Campo on April 18th and hopefully crossing into Canada on or before 31st August. I will fly back from Seattle on September 2nd.
If I make it, and that is far from being a gimme, as I am fat, fifty one and have a bad left knee, I will almost certainly lose a significant amount of body weight. Some people lose a third of it or more. I consider this a plus, but any thru-hiker knows that their relationship with the trail largely boils down to food. Its common for resupply days to be about obsessive laundering, blogs, restocking the food for the next week's walking, and as many meals in town as its possible to cram in....three breakfasts, two lunches, maybe a couple of dinners. Its basically impossible to carry enough calories to replace what's being burned off, so resupply days are very much about gorging to compensate.