Yep, there is now way a Westie needs 20 miles a day, and if it did its hips would be **** in no time.
Stop ignoring him as you have a new baby, play with him, do fetch and find it games and may’be hire a dog walker that can take him out with groups of dogs so he gets some fun, if you really don’t have *any* time to spend with him, bearing in mind you watching telly with the dog in the same room is not going to mentally simulate him.
The 20 miles is more to do with dedicated time with the dog rather than distance, and you have to remember that dogs aren’t like bike bits that you can replace once you are bored with them.
basically MTFU, give you dog some attention and stop looking for quick/lazy fixes, because you can’t arsed to play with your dog anymore.