Being so high above the sea level, most visitors suffer the "altitude sickness". In the first 2-3 days you will feel a little headache and some stomach problems. To fight that is advisable to drink coca tea and take it easy, move real slow.
If you spend 2 days in Cuzco before the trail you shouldn't have problems during the trail, but anyway, take it easy, slow, don't make extra efforts, and remember this is not a competition. You are here to enjoy the trail, stop when you feel tired, contemplate the views, sit down at the steps whenever you feel breathless!
When you walk on the Inca trail, be aware of the depths that are next to the track. It's not dangerous to walk there and you get used to the depths next to you. An advice that our guides give us several times was that you have to walk or stand still next to the hillside when people or animals pass you.
The local people use horses and mules to transport goods up and down the mountains. We came once in the middle of such a transport, a dozen horses that were carrying furniture and tiles up the hill. Look for a place next to the hillside so that a horse can't accidentally push you over the edge.
Also when faster walkers or porters pass you, keep to the hill side, because most walkers and certainly the porters carrying big packs on their backs and they are not always aware how wide they are. So a accident can happen quickly, so stay save and keep always to the hill side.
Huayna Picchu, is the 2701 meters high pinnacle that is visible in the background of the traditional postcard view of Machu Picchu. "Huayna Picchu" means in the local dialect, "New" or "Young Peak".
The views from that top are seem to be very nice, but if you have only one day, rethink if you should do that especially if you already did the Inca trail. When you go, you have to sign in and sign out when you come back so that they can see if there is some one missing.
Our guide told us that if we wanted to go, we had to be very careful because the track has some dangerous passing. So we decided to give it a miss, the views from this side are already impressive, but from op the Huayna Picchu they are even better.
Note that if you are climbing one of the smaller view peaks like Una Picchu or Vanyaya Pichu. Make sure that the trail is dry. It can be fatal if you have a fall as there is nothing much to arrest you fall.
The stairs are not even, or sometimes there is extra high steps... Also watch when there are big drops, I am sure you would not like thumbling down... Ouch!