There is a great thread here about altitude sickness @wander_tara - Machu Picchu altitude issues - #8 by OZ_TRAVELLER
@kelly - If you are active and walk plenty then I wouldn’t say you need to train too much. The ups and downs often involve many steps (gringo killers as they are affectionately named), so be prepared for that. I did the Inca Trail trek a few years ago and did no training (but walked a lot in general) and I was fine, but it all depends on the individual - everyone in our group managed, some struggled more than others. The key is - acclimatise in Cusco for a few days before hand, as it is the altitude more than the trek that might make it harder.
Overall, I would 100% recommend it, it is definitely not ‘easy’ but it is a rewarding challenge, and you will see so many views and Inca ruins you will just not be able to see if you just go to Machu Picchu. It definitely feels special being on that ancient path. Plus, you get to see Machu Picchu from the sun gate (the famous view), when no one else has entered the ruins, which is much more picturesque than when you get down there and it’s busy with tourists!