You cannot be too well trained for this hike. The better trained you are, the more you will enjoy it. The altitude is an issue for most people, and some are hit harder than others. We were the oldest people on the hike, but there were a few younger people who also struggled. I would recommend daily hill walking for at least two months before going, as well as weekly hikes or better yet, two days in a row of hiking on challenging mountain trails, carrying day packs that are about 10 pounds.
Packing tips: there were many websites that offered tips on what to bring. We especially found the following helpful: travelfashiongirl.com
The most important thing that we found was to be prepared for very cold nights, as we were high in the Andes in the Peruvian winter, and we woke up to frost on the second night. The first night was almost as cold. If you use a metal water bottle, like those sold in most dollar stores, you can ask the guides to put boiling water in them for a hot water bottle. These will last most of the night, and can be used as drinking water the following day. Take a wool hat that is comfortable to sleep in and a fleece with a hood to keep your neck warm while you sleep. I wore two pairs of winter running tights and a pair of heavy wool socks to sleep in, kept a separate "sleep shirt" that I hadn't already sweated in, and put on my long sleeved shirt and fleece jacket. We put our clothes for the morning at the bottom of our sleeping bag so that they would be warm in the morning. I had zip off hiking pants, but wore lycra running shorts under them during the day for extra warmth. Most people just wore their long pants all day, but if you get uncomfortable hiking in the warmth, you will want to have shorts and tank top. WARM gloves are essential, as both Marianne and I found the light ones we had were not adequate. Those who had heavy North Face gloves were happy. You might also want a light pair for moderately cold marnings. We did not bring hotshots for those early morning hikes, but wished that we had.
YOU WILL SWEAT......A LOT on this hike, so we recommend bringing a short sleeved t-shirt for each day (the fresh one can be your sleep shirt). I found that the all in one bra-tank top to be a good alternative to t-shirts, as I get warm very quickly, and prefer to take off as many clothes as possible when warm. Marianne, on the other hand, was quite comfortable wearing a short sleeved t-shirt through the warmer parts of the day. A t-shirt also offers greater protection from sun and bugs.
YOUR FEET WILL GET STINKY hiking all day and sweating in your hiking boots. I would suggest three changes of socks, one for each day, plus one extra warm pair for sleeping in.
Hygiene: this was the hardest part. Although Alpaca Adventures was fantastic about providing a bowl of hot water and a cloth for washing every day, you might want to bring your own facecloth and small towel. In addition, bring plenty of handi wipes so that you can clean such body parts as stinky feet each night. Personal hygiene wipes are also a good idea to help keep clean.
Suggestions from fellow hikers: bring dryer sheets to put in both clean and dirty clothes bags to keep them sweet smelling. (Think about those socks!) One woman brought a "Johnny" so that she could go to the bathroom without needing to squat, and a "Freshette" so that she could avoid the dreaded march to the porta potty in the middle of the night. These are both available on Amazon.com
Ask the tour company for an air mattress and walking sticks. They may charge extra for these, but they are well worth it. Bring a scarf for warmth, and also to use for a pillow. You may want to invest in a little camp pillow for additional comfort. (These are also great for bus and plane travel)
There are some porters who are using terrible footwear such as very worn sandals with no socks to do the hike at double the time that we were doing. If you have good walking/running shoes or hiking boots to donate, bring them with you. They will be appreciated. This is also true for warm gloves and clothes.
BE PREPARED TO GET SICK! Apparently this is very common on the Inca trail, as the tough hiking, lack of sleep and cold weather may depress your immune system. Bring cold medication, antibiotics, something for upset stomache and traveller's diarrhea, as well as something for altitude sickness. If you have done the trail and have any additional suggestions, please let me know and I will add them to the list.