A playmate may be a great solution for you.
As long as he is getting some exercise each day and he is not overweight, I would not worry about it. I personally do not walk my dogs ever. I have had ours stolen before- call me paranoid but that's the way I roll now.
We have a large yard, and throwing, chasing the ball and playing with each other, our dogs get a vast quanity of exercise. So much in fact, that when they come inside they just plop onto a cool spot on the floor and nap for a few hours.
Don't beat yourself up about it, just find what works best for you and your Winston