If you're not used to walking, then practice. Some in our group (including me) had never really hiked before and although we all got through it, we weren't expecting it to be quite as challenging. The altitude will hit you, not necessarily with altitude sickness but you will notice it so just stop, drink and take in the scenery. Apart from minor headaches from some in our group, none of us really experienced altitude sickness.
Physically this was the hardest challenge many of us in our group had ever done. Every day you will be climbing, then going down, then back up again. It was a great experience and we were very lucky with the weather as I really can't imagine doing let alone enjoying this trip in the pouring rain.
Our group was fairly large at 14, and for the most part got along extremely well. Lots of laughing in the dining tents at meal times and encouragement on the walk itself. Toilet facilities are pretty basic, but we all got over the embarrasment fairly quickly. Keep a sense of humour!
Finally when reaching the Sun Gate, don't be suprised if you're a little underwhelmed. It's not because Machu Pichu isn't stunning, it is and the scale of the site is breathtaking...but for me and others we were kind of relieved that all the hiking was nearly over (45-60mins away)!