I was a bit nervous before going on the trail about its level of toughness. In the end it was well doable even though not easy, but we were also rather lucky with the weather. I spent 4 great days on the Inca Trail and can absolutely recommend it.
You should definitely bring a pair of walking sticks with you though. You might also think about if you really want to add an additional couple of hours hiking to the trail with Huayna Picchu like me or if you just leave it alone and enjoy the site of Machu Picchu without any stress instead.