Glad you like the game.
The way I run it is that you don't get any endurance back on a failure - you loose it all: both what you spend AND the difference. This makes for pretty brutal games where players are forced to find ways to recover Endurance pretty often (this often leads to players changing goals to find a safe space and rest instead of proceeding).
I don't play with any "obstacle turns" - just swap focus between characters as it makes sense in the narrative. The enemies/obstacle doesn't get their own turn, they are a constant threat. So if two players approach a zombie, the zombie grabs one of them (ask the player to roll to avoid), then switch focus to the other player saying something around the lines "the zombie will bite into the first player's character neck, what do you do?) and so on.
Saying all that, this is how I play it, you are free to play the way you want. If introducing turns is more your style - go for it! If you or your players don't enjoy finding ways to recover Endurance in the middle of the action, feel free to give the spent Endurance back to the players :)
Hope that helps.