So onto the actual trail. The trailhead for the Doi Langka hike is at a radar station/phone tower, coordinates 19.051933, 99.381713. The bottom of this road has a barrier across it but motorcycles can just drive around the barrier. If you had a car the barrier is not locked so you could lift it up and drive through. In my experience the radar station at the top is not manned and I think it unlikely you would encounter anybody on the road or at the top. If you did not drive through the barrier you would be faced with a 3km uphill walk just to get to the trailhead. Of course if doing this trek with a NP guide they would drive you to the start.
Since I was arriving by motorbike and not using guides I needed to get back to my bike, so I turned around at Doi Langka Noi, camped at Doi Langka and finished where I started. Total distance over 2 days about 30km. In doing this I had the benefit or losing a lot of gear weight once I reached Doi Langka as I could set up camp there and return to it later. With just essentials on my back progress was fast to Langka Noi and back. Here is my GPS route trace (one way only, the return journey). The white line is Doi Langka Noi to Doi Langka Luang and the blue line is campsite at Doi Langka Luang back to trailhead.
The white line is a moderate difficulty, mostly gradual inclines and only one or two steep climbs/descents. Half of it is in the forest but you are never too far from a ridgeline or good view. The blue line is always ridgeline, very technical and steep throughout; you are always going up or down and the inclines are always steep, trekking poles or a good stick help a lot as they move some of the strain away from your legs to your arms and also help with balance. Especially with a big backpack I would say the blue line is about as tough an 'ouch' trail as you can find. Elevation profiles:
You can see the steepness between Langka Noi and Langka Luang is not as harsh as trailhead to Langka Luang (remember the elevation profiles are in reverse - back to the start).
The entire 19km trail (as well as the Gurgling Trail) is marked on the Backcountry Navigator app but here are my gps files anyway in case you want to use a different app:
Since this is mainly a ridgeline trek, there is no natural water source until you get very close to Doi Langka Noi, which is a couple of days/14km from the start, so you *may* need to carry a lot of water. I say 'may' because when I went, there were large bags of water at each of the campsites, but I think that they were left over from the trail race I mentioned, which had just happened a day before. I did this trek in December so a trek in March or April may not have any flow at all, and a trek in the middle of the rainy season may have a few more streams.
It is cold on any mountain in North Thailand in winter; 10 degrees is normal and it can go as low as freezing. I had a brilliant night's sleep (9 hours) in a 10 degrees rated sleeping bag, 2 pairs of socks, 2 t-shirts, one hooded top, thick gloves and 2 pairs of trousers. This all adds to the weight you will have to carry though, your pack will probably be over 10kg including tent, food, water, sleeping bag, clothes, camera, torches and powerbanks/batteries. If trekking with a guide you can usually arrange porters who will carry your stuff for a fee. When I woke up the thermometer somebody has nailed to a tree read 12 degrees, but by then the sun had risen. In the night I think it would've been about 8.
2000m is a high elevation and less oxygen makes any exercise harder. It's only a few km a day on paper but when you add in the inclines and thin air; the reality is tougher. Due to the overall distance and the amount of stuff you need to carry (particularly water), I wouldn't recommend the full 19km trek or my version of it without a guide unless you are experienced trekking and in good shape. Even though this trail is well-marked and fairly obvious throughout, you should still have a gps (or phone with gps app). I didn't see anybody else on the trail at all over 2 days. Nor did I see or hear any animals (or any tracks) by the way, except birds and insects. I noticed a lot of the trees were scorched so recent forest fires may have had something to do with that but more likely the forest has just been poached to death as is the case in most of the north.
My legs took about 3 days to recover from this trek! Eat lots of protein to speed up muscle recovery.