Firstly; go in the off season.
Second: don't book up front.. just go there.. there are lot's of companies all around the main squares in cusco where you can book the inka trail hike from.. People from all over south america used to tell us that it would be hard to get on to them if you didn't book.. but that was bull****.. if we booked it would've costed us like 400/500 dollars.. while going just to a tour agency and get it 'on the fly' costed me 180 dollars..
Yes, the inka trail is intense.. especially the first day.. but from the 2nd day on we started doing cocaine along the way and it all worked out
just keep in mind that you keep going up steps which could be like a half a metre each.. it get's into your legs after a day..
Oh and.. be sure to tip the people who accompany you well.. these guys are insane.. they run the trails ahead of the group to set up camp and such.. you should see them go.. it's amazing