I recently completed the 5-day treak to the Lost City, and I thought it was my favorite travel experiences. It merges several of my interests: history, the outdoors and learning about new cultures. In terms of advice that would matter to you:
1. Tour group: I booked with Expotur. All of the tour groups charge the same price (600,000 COP or ~$300) regardless of whether you a 4-day, 5-day or 6-day trek. Expotur was extremely helpful answering all of my questions about the trek over email before I arrived, and they even picked me up at my hostel to go to their office to pay for the trek. I was able to use a Master Card, and there wasn't a surcharge or additional fees for using a card. Once we got to the trailhead, the tour group you chose didn't really matter. We were merged into one tour. There were 3 of us who booked through Expotur and about 12 who booked through Magic Tours. Our guides worked together, and we traveled together. The only time you'd notice we were part of different groups is that we left on different jeeps.
2. Tour guide: you can request a specific guide. I had a good experience with Miguel de J. Gallego. He was friendly, had plenty of information about the trail and the city, and he went out of his way to show us interesting spots to swim. On the downside, he didn't speak English, but none of the guides did. Luckily, there were enough multilingual people in the group to translate.
3. How long to go for: I took the 5-day trek, and I was happy with the decision. At first I was worried, that I'd have too much down time each day because the hiking seemed to end around 2:00 each day. In reality, it typically starts raining around 2:00 anyway (even during the dry season), and you wouldn't want to be hiking anyway. Also, most of the stops had swimming areas nearby, which is a nice way to unwind. Also, even 5 days was a very tough trek. I run 3x a week and hike a lot, and I thought this was a very challenging trek. I'm not sure I would have wanted to go even faster than we did. By the way, not all of the tour groups mention that 4 days is an option (Expotur didn't), but it is an option if you ask for it.
4. What to bring: the key is to pack light. You're hiking through steep, wet terrain in 90-degree weather. A heavy pack is a major problem. You'll need at least 2 changes of clothes. Some people wet their clothes at streams to clean them and let them dry overnight, but they never do. Even though it's hot, it also humid. When I clipped clothes to my pack during the day for hike, they were usually dry when we reached our next camp. You'll also want plenty of insect repellent. It wasn't a major problem for me, but plenty of other had big problems with mosquito bites. Another thing I found very helpful was Vibram shoes. They gripped better than my hiking boots on steep inclines and declines that made up most of the trek, and you can wear them in the water so you'd don't have to take anything off for river crossings.
5. Other things to expect: every camp had showers. They weren't 5-star hotel quality, but you could clean yourself each day. Also, I had read that the Lost City wasn't very impressive, but I thought it was. Sure, don't expect to see Machu Picchu, but it's a pretty impressive site that's more than just the 3 mounds you keep seeing in pictures. Finally, safety wasn't a concern. That's a bigger issue in the southern parts of the country, and there were military posts with soldiers at the start of the trail, and at the Lost City itself.
Like I mentioned, this trek was a lot of fun. It was a great workout, I learned a lot, and I met incredibly interesting people from all around the world. I'd definitely recommended it to anyone who like hiking and culture.
This review is the subjective opinion of a TripAdvisor member and not of TripAdvisor LLC.