The park is the third largest in Thailand. It covers an area of 300 square kilometers, including tropical seasonal forests and grasslands. Its altitude mostly ranges from 400–1,000 m above sea level. There are 3,000 species of plants, 320 species of birds, and 67 species of mammals, recording in this, Thailand's most famous National Park.
There are two trails noted here, of 4 km each, they are 1) Km 33 to Nong Pak Chi and 2) Nong Pak Chi to Wang Jumpee. The first of the two treks is a simple trail with some absolutely stunning examples of different species of fig (ficus) trees. Its an easy walk and takes about an hour.
The second is a little tougher with the 4 km Wang Jumpee trail a bit more wild and passing through a wetter forest type (consequently leach infested and with a higher localized humidity) on the hill climb section. Both trails can be done together for a good day out with a moderate hike.
The Km33-Nong Pak Chi trail can be done on its own as an "easy" walk (for definitions of "easy" and moderate" see the sticky post giving the descriptions. The Wang Jumpee trail would probably earn a moderate label due to the hill climb, steam crossings, humidity and the leeches.
I've recently been walking in this area, planning to get out at Wang Jumpi, once started from Nong Pak Chi, and once from what they call Dong Tiew, but in both cases - by following the clearest trail - I ended up at the Ton Sai saltlick area. Not really a problem, but anyway. I've walked the original trail in the past without problems, but I guess due to some fallen trees the trail to Wang Jumpi got obstructed. I'm quite sure that when starting at Wang Jumpi you'll manage to connect to the same trail. If I have time I'll try that to find where exactly the trail branches off to Ton Sai. If you do arrive at river near Ton Sai, you can always walk along the river (upstream) to the Wang Jumpi river crossing. Another option is to follow the river downstream to the new picknick area behind those 5 small restaurants. Even though it's quite close to the road, the wildlife doesn't seem to mind. But that seems to be the case just about anywhere in Khao Yai.
We had a similar issue, when we started the trail at Wang Jumpi the trail was straight forward, when we came in the other way (not following the GPS but just the visual trail) we ended up coming out just upstream of the food centre. There were some confusing tree markers as well for small trails that just seemed to disappear.
We also tried Mo Singto to the old TAT restaurant back then but there was so much storm fall (it was middle of rainy season) that we did not make it. I need to go back one day and add in that track.