Long hikes not possible with Fenix 5S plus?!

The official specs from Garmin says that it can run up to 11 hours with GPS.

If you need 10 hours, you don't buy a device which only promises "up to 11 hours".

"Up to 11 hours" basically means that in some theoretical use case which will probably not fit your use case, you can reach 11 hours. In all other use cases, you can be certain that it will last shorter than 11 hours.

As Allan says above, the spec is up to 11 hours of GPS use per charge.

If you require over 10 hours of use, you could check out the ultratrac mode, or you should probably have bought a watch with a bigger battery.

I'll correct it a bit: my 40 km is about 9 hours so I had to mention "up to 10 hours". 

Lucky me I use this Garmin mostly for runs, the longest has been 4 hours. Thanks for the replies. 

What you could try is to disable GPS and enable the UltraTrac mode. In this mode you will have up to 25h (according to specs). The Ultratrac mode collects data less frequently where pace and exact distance are not that important such as with your hiking activity or ultra running. 

True, but you know what I mean.

Hm. I (and others) found the UltraTrac mode pretty useless. If it would record GPS just every minute or so, it would make sense, but the way how it is implemented (prediction of distance taking direction with current pace), leads to tracks that are so wrong that they are completely useless.
Check this thread: forums.garmin.com/.../some-thoughts-on-ultratrac-mode

Try running that 40km instead of walking :)

I did 42,195 km 3 times :) 

Nice work!