I just don't know to do

Sounds to me like you do NOT have the necessary time nor inclination to provide for that high of an energy dog.

Find a home with younger outdoor types.

Having bad knees makes it difficult to provide that needed involvement.
I know I have bad knee and foot so our rescue dog is older mellow slow walker because that is what I can provide for.
No shame..just life.
Do the dog a favor and place with energetic homebodies

First, talk to your doctor about your knees. It may be something as simple as a shoe insert. Or it may be something that walking will help.

If your doctor gives you the OK for walking, take him to an obedience class. It's once a week normally and will help him socialize with other dogs and people - invaluable. Then, each night you are not at obedience class, spend 1/2-1 hour walking him and practicing what you've learned. It is not running, etc., and will likely help your knees.

You say "we". If you can't take him out for obedience and socialization, then make it the responsibility of one of the other family members. That's the great thing about obedience training - it doesn't require an adult. A child 10 or older can easily learn obedience and master walking the dog. That would allow you to sit and watch while they do the work.

Terriers are thinkers. While physical exercise is great, they do best with mental exercise. Do a search on "mental stimulation toys" for dogs. There are lots of different options. Those toys while you are at work can help a lot.

Golf cart or bicycle are not an option. Golf cart - we are in town and can't use one. Bicycle, one or both of us would end up run over. The chuck it would be great except it is winter and I get home after dark. We have spent yours throwing a tennis ball in a local ball field in summer when it is still light when I get home. He can spend two hours straight running tennis balls in a field.
Edited for spelling.

He is a mutt. Probably a terrier of some kind mixed with lab. He looks a lot like a miniature lab (even had webbed feet) but acts like a terrier.

We have seen people riding in their golf cart with the dog ( usually a large dog breed) walking then running for a short distance for exercise. The dog was on a 6 ft leash so was able to be well away from the side of the golf cart as they trotted along. I wouldn't do this with a small dog though.

Along side a bike may end up with both dog and rider in a world of hurt.

I second the Chuck-It, it's a great invention. I understand the difficulties, but they can be managed with time (I know that's an issue) and training.

Off the top of my head, trotting him alongside a bicycle; finding a big field and investing in a Chuck-It (long-armed thing that lets you throw a tennis ball about 1/4 mile...LOL!) or teaching him to walk on a human treadmill....might give you a way to exercise him and not abuse your own body?

Disclosing the dogs breed would be very helpful.