Hi guys, I’m relatively new to cycling as I never learnt as a kid due to knee problems, any advice on training or how to build up my legs and endurance?

The most important thing by far is to approach it in the same way a kid would, just ride for fun. Kids don’t build up legs or endurance, they just have a laugh.

Go out and ride, have fun, ride some more, have more fun. Then, start riding with other people, local clubs or forum members, making sure that you are always having fun. Over time legs and endurance will just happen.

Just enjoy riding bikes.

get some skills training early before you develop bad habits.

as an adult you won’t bounce when you fall off like you would have done as a kid so the better your technique the less you’ll make silly errors that cause problems.

You’ll still fall off, obviously, just with more style.

Get someone to show you how big your bike needs to be and high your saddle needs to be (a good bike shop), that’ll be one of the biggest issues you’ll face in ensuring you don’t cock up your knee again. You’ll be amazed at how many ‘experienced’ riders have their saddle at the wrong height.

Take it nice and easy at the start, make sure you rest between rides.

And as above, spend as much time enjoying things as you can, the fitness will just happen.

Just get out and ride and don’t be put off by the aches from the first few rides, it gets easier with time. Oh and make sure your saddle is set at the right height, I see loads of newbies struggling unnecessarily because of this.
Good luck

Edit : what he said ^^^