Hi Radek

A few thoughts for you. A coastal trail doesn't always mean it is flat. Imagine if you were hiking in and out of a stadium over and over. You would have to go up steps and then down them, again and again. You would never reach a high elevation, but you would be doing a lot of climbing. just saying...

Also--if you can buy food on this route as you hike along, or even every few days, then 40kg is a very heavy pack. I think most of the people here would carry something between 8-12kg if you don't include food. All you need is a tent, a sleeping bag, maybe a warm jacket and a rain shell, and the pack itself...and the food you can buy. Even with food for eight full days on the trail, my pack weighs about 15kg.

20km is perfectly reasonable, if the trail is in good condition. If it's hard to follow, very steep or rocky, or even if it is in the sand for a long time then you may need to aim at fewer km....but I think your plan is perfectly reasonable...