Treadmill vs Apple Watch when exercising

I’ve noticed when exercising in the mornings I do 2 hours on my treadmill going 3.5 m/ph and when down it says I’ve walked 6.99 miles just shy of 7 miles but my Apple Watch says I’ve only done 5.4 miles

2 hours Apple Watch says I did 12,230 steps burning 545 calories but the treadmill says I’m burned 545 calories, so I’m burning the same calories on the treadmill and warship but my watch lost me 1.5 miles.

When I’m on the treadmill going 3.5 m/ph my watch says I’m only going 2.4m/ph how can I fix this problem?

I’m using my Apple Watch SE