You should average between 2 and 2.5 km per hour - so 90 km is between 36 and 45 hrs of actual walking - add on time for breaks, lunch and swims etc - so, say about 6 hours of actual walking per day comes to between 6 and 8 days.

If you have been training, you should know how fast you walk through the bush and be able to work it out, but allow for steepness and slipperiness.