As long as he keeps moving and has a warm coat on he should be fine. If he stops moving, that's when he will get too cold. I always feel the ears during a very cold day to check that they're still warm. If the ears are cold then it's time to get him indoors. Of course there will be days when it's far too cold to be outside for longer than a potty break. Those were the days I NEVER left him alone. He was quite happy to snuggle on the couch under a blanket with me watching TV all day.
I picked up a warm coat at a local pet store during the end of season sales but it was still expensive! If you have a tack shop near you, they usually carry horse coats sized down for dogs that are around $40. There are also online sites that sell dog coats however I've never gone that route. I like to see them and fit them to my pup.
Yes treadmills are insanely expensive
That's why I added a third walk and the excercise was great for me too. I have to admit I've put on a few pounds in the last year
