The inca trail is an unforgettable experience; I did it back in September 2006 and if you are reasonably fit, then you should have no problems. The thing that can make it quite difficult is the altitude and to be honest, even super fit people can be affected by it badly. The way to combat this is to spend at least 3-4 days at high altitude before the start of the trek so you will acclimitise, drink plenty of water, keep alcohol to a minimum (or better avoid it altogether) and most importantly of all pace yourself on the trail.
I went on a two week tour of Peru which included the Inca Trail/Macchu Picchu which started on the 7th day. On the 2nd day we at Lake Titicaca which is at 3800m which is comparable to the altitude of the Inca Trail. I remember, that we stayed on one of the islands, at 4000m, the local children challenged us to a football match which absolutely killed me due to the thin air; I was sick as a dog afterwards. So lesson learned, spend plenty of time at altitude before any signifant excercise. By the time I started the Inca Trail 4 days later I was well acclimatised.
If you join a tour (quite difficult to get permits to trek independantly); usually the cost includes the cost of porters carrying all your gear and tents/cooking facilities etc and all you carry is what you need for that day e.g. water, snacks, suncream, camera (a must).
Best times of the year is June to the end of the year as it is the dry season, though that didn't stop it raining when I was there in September and my tent being flooded out. Also it was quite disappointing on the last day to reach the "Sun Gate" overlooking Macchu Picchu (you get up 2 hours before dawn on the last day) and the whole area being covered with mist. It did clear up later and we enjoyed the ruins immensely but we didn't get to see the sunrise which is supposed to be spectacular.
I would agree with the other posters who said that the trail is a bigger highlight than Macchu Picchu but it is still a great reward when you get there. The scenary is stunning and there are loads of Inca ruins along the trail to explore.
I went with GAP; the tour was Peru Panorama (see link below) a 15 day tour which covered Lake Titicaca, Inca Trail/Macchu Picchu and the Amazon Jungle which is an all round great tour. You can also get other tours which would also include Colcyn Canyon and Nasza Lines which apparently are also well worth it if you have the extra time. You can also get tours which will visit Macchu Picchu without the Inca Trail but you are really just cheating yourself if you did this. There are also other trekking options like the shorter 2 day Inca Trail but again if you take the trouble to fly half way around the globe you should go the whole hog. Oh if you are really adventurous, there is a 15 day trek which incorporates the Inca Trail over the snow capped Andes but you have to be more technically experienced for this as the use of crampons etc is requird
I am a bit on the chubby side myself and don't exactly do loads of exercise (although I did plenty of walking (not mountains) ahead of the trip) and I managed it; so it really is more of a case of "mind over matter". The guides allow you plenty of time and keep the pace slow so there is nothing really to worry about, but I will stress again ensure that you join a tour which will allow you plenty of time to acclimatise ahead of the Inca Trail.