Someone on the PCT email discussion list posted some math he did that suggested that if you are to complete your hike from Mexico to Canada you had better go out running from day one. I thought his calculations were a little unreasonable, especially since he seemed to assume that people who ease into the hike do so by hiking 10 mile days.
Most people are able to hike more than 10 miles per day after the first week, and even within the first week. You are almost forced to do this, adapt or die of thirst basically.
Hiking lots of miles is just a matter of how many hours you want to keep walking. It's not a matter of how fast you go. So to increase your miles you just have to hike more hours. This is actually quite easy. You can walk only 2 miles per hour and after 10 hours of walking, you will have gone 20 miles and still have plenty of daylight. The real challenge is going less than 20 miles. You aren't on a camping trip. You're on a hiking trip. The difference will become apparent once you get out there.
The only kind of shape you need to be in is the kind of shape where you know you can put on a pack and hike all day. It doesn't have to be any more than that. If you can do that but you are 100lbs overweight, that's ok. If you can do that but you are a slow-moving old lady, you'll be just fine.
People go home early on because of acute blisters or other injuries or because the trail wasn't the experience they expected or wanted. Because the trail is so very consistent in the grade and in the way it is laid out, all it takes is a little blister and you start favoring your foot and then you start walking a little off kilter and then the repetitive condition of the trail starts beating that little off-kilter gait into your bones. Next thing you know, your knee is shot or you have even worse blisters or your back is whacked. Eventually you may be totally ruined. Even more than being in tip-top condition, you have to have awareness of the little problems and take care of them before they snowball.
I am 45 years old. I'm female. I'm not super human by any measure. I did not hike the entire trail in one season (I did it in two), but both seasons that I did the trail, I was able to easily hike between 20 and 35 miles per day. Usually right around 28 was a typical day for me.
So, yes, get in shape as well as you can, but don't sweat it. Hiking the PCT is not a super-human endeavor requiring you to grit your teeth and go all out like a race horse. Even more important than completing the whole trail is the experience of living 3, 4, 5 months out there on the trail. It's an amazing and wonderful lifestyle. Since coming home, it seems most things about our "normal" way of life are just wrong. We are meant to move our bodies and be out in nature. Even if I had never seen the Canadian border monument, I would still have had that wonderful awareness of being fully alive. Completing the trail is only one reward. The others are much bigger.