DH and I did it 7yrs ago and it was fab. We did it with a company called SAS who seemed to have the best track record for looking after their porters (they are amazing men who can get treated like shite by some tour operators). Make sure you have cash to tip them at the end.
We'd been at altitude for a good while when we started the trip. One guy had flown in the day before and went off sick after about two hours! But, you really never know how the altitude will affect you, so just make sure you follow the advice give to you by your company. We chewed coca leaves but they just tasted horrid and I'm sure did nothing to help!
We had most of our gear carried for us, we just had basics we needed to get us through the day in our backpacks. We hired extra warm sleeping bags and bought alpaca longjohns at the local market as it gets very cold at night, and we were toasty warm in bed. We also bought cheap walking poles from the market too which were very much needed on the second half of the trek which is pretty much all downhill and plays havoc on your knees!
Fitness-wise I didn't find it as hard as I was expecting. I'm not particulary fit, but it was ok, I've done harder treks. But it is a good idea to get fit before as it's a long way and there are some steep climbs so every little helps.
On our last night on the trail we were given the option of paying to have a shower, or visiting Huinay Huayna. Only a few of us skipped the shower to go to the ruins, which were breathtaking. Showers can wait!
You'll love it. Peru is an amazing country with so much to see, and Macchu Picchu is one of the new Wonders of the World for a reason 