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.

How long are your breaks? How much time do you spend waiting for traffic lights or letting other vehicles pass? What's your standard speed?

Maybe you could go faster if you did shorter distances. 3 hours is kind of a long time for doing a sport you only picked up 6 weeks ago.

Are you riding on pavement or trails? Mountainbike tires are generally a lot slower on tarmac, due to the tire pattern, and width, compared to a road tire. Also on tarmac you should increase the pressure in your tire quite a bit.

Don't listen to which speeds people tell you to aim for. Look for improvements in your own rides.

Road surface, hills, wind, tires, style of bike, your height, position on the bike, traffic and many more things have an impact on your speed. Compare yourself with yourself