Your Garmin help, please

Very excited to be getting my first garmin, decided on the edge 800.

What's the battery life like realistically, would I be foolish to expect it to be able to use strava and maps for a 100 mile ride?

Depends how slow you are. You should get around 12 hours out of it with mapping enabled and some screen switching, messing with settings, odd recalculation, etc.