As an (ex currently post child..) mountain biker my threshold for having good endurance or not was being able to ride at pace for hours without sitting down. It takes a lot of fitness to do, but I found when you train like that it comes fairly quickly.

I say that because riding standing I found let's you put more power down and also makes using minimum suspension comfortable as you use your knees instead. You need to find what works re bars and stem though as you can fuck your back up in the wrong position. To ride xc or full sus bikes off the trails I had a second set of wheels with smooth road tyres. Anything but road tyres will dramatically increase your rolling resistance on tarmac.