Among the many roads and trails constructed in pre-Columbian South America, the Inca road system (El Camino Inca) of Peru was the most extensive. Traversing the Andes mountains and reaching heights of over 5,000 m (16,500 feet) above sea level, the trails connected the regions of the Inca empire from the
Inca empire from the northern provincial capital in Quito, Ecuador past the modern city of Santiago, Chile in the south. The Inca road system covered approximately 22,500 km (14,000 mi) and provided access to over three million km² of territory.Because the Incas did not make use of the wheel for transportation, and did not have horses until the arrival of the Spanish in Peru in the 16th century, the trails were used almost exclusively by people walking, sometimes accompanied by pack animals, usually the llama.

The trails were used by the Inca people as a means of relaying messages, carried via knotted-cord quipu and by memory; and for transporting goods. Messages could be carried by chasqui runners covering as much as 240 km (150 mi) per day, working in relay fashion much like the Pony Express of the 1860s in North America.
There were approximately 2,000 inns, or tambos, placed at even intervals along the trails. The inns provided food, shelter and military supplies to the tens of thousands who traveled the roads. There were corrals for llamas and stored provisions such as corn, lima beans, dried potatoes, and llama jerky. Along the roads, local villagers would plant fruit trees that were watered by irrigation ditches. This enabled chasqui runners and other travelers to be refreshed while on their journeys. Inca rope bridges provided access across valleys.
Many of the trails converge on the center of the empire, the Inca capital city of Cuzco. Therefore, it was easy for the Spanish conquistadors to locate the city. Traversing the trails on horseback proved to be difficult and treacherous for the Spanish in their attempts to conquer the Inca Empire

Main Routes
The most important Inca road was the Camino Real, as it is known in Spanish, with a length of 5,200 km (3,230 mi). It began in Quito, Ecuador, passed through Cusco, and ended in what is now Tucumán, Argentina. The Camino Real traversed the mountain ranges of the Andes, with peak altitudes of more than 5,000 m. El Camino de la Costa, the coastal trail, with a length of 4,000 km (2,420 mi), ran parallel to the sea and was linked with the Camino Real by many smaller routes.
Inca trail to Machu Picchu
Much of the trail is of original Incan construction.
By far the most popular of the Inca trails for trekking is the Capaq Nan trail, which leads from the village of Ollantaytambo to Machu Picchu, the so-called "Lost City of the Incas". There are many well-preserved ruins along the way, and hundreds of thousands of tourists from around the world make the three- or four-day trek each year, accompanied by guides.
The Inca Trail to Machu Picchu is actually three routes, which all meet up near Inti-Pata, the 'Sun Gate' and entrance to Machu Picchu. The three trails are known as the Mollepata, Classic and One Day trails, with Mollepata being the longest of the three. Passing through the Andes mountain range and sections of the Amazon rainforest, the Trail passes several well-preserved Inca ruins and settlements before ending at the Sun Gate on Machu Picchu mountain. The two longer routes require an ascent to beyond 12,000 ft (3,660 m) above sea level, which can result in altitude sickness.
Concern about overuse leading to erosion has led the Peruvian government to place a limit on the number of people who may hike this trail per season, and to sharply limit the companies that can provide guides. As a result, advance booking is mandatory. A maximum of 500 people, including guides and porters, are permitted to begin the trail every day. As a result, the high season books out very quickly.
Note that the trail is closed every February for cleaning.
The Classic Trail (four-day trek)

The four-day trail or Classic Trail starts from one of two points; km 88 or km 82, on the Urubamba River and 88 km and 82 km from Ollantaytambo. The first day is relatively easy, covering no more than 13 km in a few hours, passing by the Inca ruins of Llaqtapata, a site used for crop production and which has remained well preserved.
Day two includes the ascent to Warmiwañusca or Dead Woman's Pass, which, at 4,215 m above sea level, is the highest point on the trail. Day three starts with the final climb to Dead Woman's Pass, although some groups climb to the top of the pass on the second day and camp 600m below it on the other side at Pacaymayu. The views from the top provide excellent views of nearby mountains such as Salkantay and Veronika. After a second pass is the site of Sayaqmarka, perched atop a sheer cliff. After Sayaqmarka the Trail continues through thick cloud forest and jungle, filled with tropical flowers and colourful orchids. The third and final pass is Phuyupatmarka.
The final day sees a descent past Wiñay Wayna, an impressive and well-preserved Inca site, where the one-day trail meets up with the main route.

Preparation - How fit you need to be

To trek you do not need to be an Olympic athlete nor a mountaineer but it is important to be relatively fit and in good physical condition before you start the Inca Trail. A few weeks of training, prior to arriving in Peru, will enhance your experience.
Try to spend an hour a day on the road. Walk upstairs rather than taking the elevator, if possible, walk or cycle when you would normally drive or ride a bus, take the dog for a walk around the neighborhood. Better yet, go on hikes in your area.

Being able to run a few miles each day without issues is probably the best single physical activity you should consider. Other advice we heard was to spend time on the stepper; we can't argue with that.
While you are training you can also be breaking in those new trekking boots that may otherwise give you blisters on the first day of the trail.
Arriving in Cusco a few days early is also highly recommended. High altitudes affect everybody in different ways, even a marathon runner may feel debilitated. When in Cusco, go visit some ruins in the surroundings, have a little jog, you will probably notice heavy breathing. This is due to the thin air at altitude, not your lack of fitness.

After a day or two of aclimatization, you'll learn how much food your body can handle in a day, whether coca tea helps, or if acetazolamide is appropriate.