Santa Cruz trek

I'm heading to Peru in June and I am planning on going to huarez to do some hiking for a week or two while I'm there. I want to do the Santa Cruz trek and I see some places online where you can book a trip with guides. I'm just wondering if you typically would have to book this is advance or can I just go to huarez and find a touring company in the first few days and plan it with them when I get there?