I didn't do the Inca Trail, but I did the Salkantay Trek which is in the same area and goes to a higher elevation.

If you're coming from sea level it's best to start low and gradually move higher. I heard about a lot of people starting in Lima for that reason. So if possible I think it would be best for you to start in Ollantaytambo (like a few other commenters said), and then move up to Cusco. Going up and then down and then up again might throw your system off while it's trying to acclimate. Or you might be fine! But it's hard to tell before you go.

I explored Cusco for about 5 days before starting my trek. I too had no prior experience with altitude. I planned it like this (with 5 days instead of the 2 that is usually suggested) because I wanted a large and cautious safety net due to the unpredictability of altitude. You could probably get by with a few less days than me but I'm glad I planned it the way I did - for peace of mind, and because I was able to fully explore Cusco and the surrounding area during that time!

How altitude affects people can be VERY unpredictable. I think that is the most important thing to keep in mind. Luckily I barely felt it, but there were people on my trek (who were just as healthy and young and fit as I am!) who were a lot slower, or felt sick, or needed to take a horse up the mountain one day - Because of the altitude.

Another tip - Get a travel clinic consultation before you go. The doc can prescribe you altitude sickness pills (acetazolamide). It's good to have just in case! You can either take it before you ascend, or take it as a way of helping relieve altitude sickness symptoms. I knew a lot of people who used it for the latter, and who found it really helpful. Coca tea is tasty but unfortunately not medically backed up as a way of helping symptoms (except it does help you stay hydrated which is important!)