Using maps: how affects this your battery life?

The Instinct doesn't have maps.

Displaying maps on screen eats a chunk of "extra" battery, but you can always set up your data pages in such a way that you don't need your map on screen all the time. 

Instinct indeed no maps. I meant breadcrumb. I walk sometimes 100 miles on my own, so i need the whole route navigation.

Yes, but if you create a route and use navigation, you can set up several data fields on your main page to see when it's time to look at the map. That's what I do, at least.

Next Waypoint 

Distance to Next Waypoint

Time to Next Waypoint

Next Fork

etc. 

You'll also get turn by turn prompts. 

Having a map on your activity doesn't inherently reduce the battery life at all - but having it DISPLAYED will reduce battery life. The solution is simple - just scroll away from the map whenever you're not actively looking at it. Also set the map orientation to North Up so the map doesn't have to constantly redraw itself as much. If you do that, then the battery impact of having maps is negligible.

Navigation (having a route loaded) does also use more battery as well, and that's true whether or not the map is displayed. In my experience having a route loaded makes it lose a couple hours of GPS time.

If you need it for 100 mile ultras you should consider Fenix 7X which has significantly larger battery.