1) I went on the trip over Xmas and New Year and, although it may seem too obvious to mention, if you are going to go on this trip at this sort of time of year DO expect to encounter some VERY cold temperatures and pretty extreme weather in general. Take a sensible amount of suitable, warm clothes as, if you are cold, you are not going to enjoy this holiday.
2) The circuit takes in many suspension footbridges, These vary greatly in quality (some look very flimsy) and height from the ground/water. I am not too keen on heights and so, I was very anxious when crossing some of these bridges. I would imagine that people who have an extreme fear of heights would find crossing some of these bridges VERY difficult and maybe even impossible.
3) Upset stomachs (either from bacteria or the effects of altitude) are common. Be prepared with suitable medicines!
4) A lot of the tracks are very dusty. It would be a good idea to bring some sort of "surgeon's mask" or something like that. A lot of the locals wear them so you don't need to worry about looking like a weirdo.
5) Watch out for the donkeys! They don't care who or what is in front of them. They will just barge you out of the way and don't stop!
6) The circuit does involves some strenuous hiking. I would say, to be up to making this trip while enjoying (rather than enduring) the walking you would need to be averagely fit if you are in your 20's, a little above average if in your 30's. significantly above average if in your 40's, and so on.