Drink loads! Even it you don't feel thirsty or hot. I suffered from dehydration on one day because it hadn't been as hot and I hadn't found the walk that day as strenuous and so didn't drink nearly enough. And coca tea, even just cold in your flask really does help with the altitude!
Make sure you've got plenty of wet wipes and hand sanitiser with you, and insect repellant even if you're only doing the Inca Trail - you can get pretty bitten on the last day of the trail
Go at your own pace you'll find it easier to manage the effects of altitude and you'll appreciate the scenery more!
If you're a light sleeper, perhaps take your own thermarest mat - the ones we were provided were adequate but a little thin for me as I could feel the bumps in the ground
I had been wanting to do this trip for years but being unable to convince friends to go with me, I decided to go on my own. I didn't once feel uncomfortable about being a solo traveller and although you can opt for a single supplement if you are travelling alone, for me it added to the experience sharing with someone.
Exodus were excellent, from the booking of the trip to seeing us on the plane in Cusco and we were extraordinarily well looked after, especially on the trail. The food was excellent, and our guides and porters were very caring, especially if we were feeling a bit under the weather due to altitude.
It was the most amazing trip; I only feel my words wouldn't do it justice if I tried to explain any more! If you aren't quite sure, just go for it, you won't regret it!