The Inca trail itself is very challenging but once you start you'll discover some strength and some determination that you don't know you have.
Doing this on your own would be impossible, but you are supported by the amazing team of Inca Trail Reservations : guides, porters, cooks. I'll be honest: initially I thought it was expensive, but considering the amount of people involved, the amount and load of work they do, and the quality of their service, I would call it a cheap price for a very intense experience.
Don't think, just book it: you will not regret it.
You will love it.