If you exercise regularly and don’t mind the heat then walking to La Ciudad Perdida will be another adventure to add to your list. For those concerned about their fitness, most of the walking days consist of 4 hour hikes on the 5 or 6 day trip. You’ll have plenty of time to rest each day.
The river crossings can be chest high but the cool water will probably come as more of a relief than anything.
You could be slipping up and down muddy tracks in the rain so if you have knee problems you may need to take extra care.
There are 1,200 oddly spaced mossy steps leading up to the entrance of the Lost City. These can be slippery. It can be a tiring walk up but you can take your time and enjoy the atmosphere as you approach your final destination.
Conditions on the track can be tough but they are not technical. It will be hot and humid during the day. Most of the time the humidity will build during the day then a torrential downpour will start in the afternoon. The rains will turn the track muddy but most of the time you will already be at your campsite.
You will cross the Buritaca River around 20 times. Sometimes you will be walking along the top of a steep section of the valley leading to the river. Your clothes will probably be constantly damp. There will be mosquitoes around but no malaria or yellow fever to worry about. Most of the time you’ll be sleeping in a hammock but there might be the option of a bed if you feel more comfortable lying flat.