What battery life should I expect?

No that's not right at all.  I would contact Garmin.

Not more than 5%! 

depends also which software version you are using curently ! Many people after updating to 25.10 had battery drain issues. In that case i would suggest you to return to 24.10 version. 

Thanks guys. I did a 2 hour walk today and used the Track Me function. Thats used 15% battery. :(
Ive found some Garmin vids on conserving battery and they say to disable Pulse Ox, but when I checked it was already disabled.

Do you think I should contact Garmin or the seller?

IMHO start with Garmin

Tufty said:

I have GPS on so I can follow a map.

If you mean that you accessed the "map" training page, you should note that the fenix has a MIP display. The battery consumption of the display increases the more pixels have to be updated per second. A static display consumes virtually no power. A map that is zoomed in a lot consumes a lot of power when you move and the pixels have to be constantly rebuilt. You reduce the power consumption a lot if you only switch to the map page when necessary. If you show the training page "time", you reduce the power consumption, because the pixels are updated only once per minute.

Do you have the normal Fenix 6 Pro (not the S and not the X version)? Are  you playing music while on activities?

So a 2h run used 70% battery and a 2h walk uses 15%? What did you do different?

Yes, I should clarify that I do listen to music while running. (although I didnt yesterday becuase I was walking). I also have the map screen open most of the time. I hadnt appreciated the fact that the more times pixels change on the screen, the more battery it uses! So I will experiment by only using the map screen when necessary. 

Ive also just found a 'battery saving mode'. I only wear my Garmin when Im running/cycling/walking - about twice weekly - the rest of the time its just sat on my desk. So Im going to turn battery saving mode on when Im not using it.

Music, the map, active navigation, live track: all of this eats additional battery.