It's very easy to forget about your egg collection — or not realize the gravity of the rewards they grant. Periodically check your egg menu (the main Pokéball icon → Pokémon → Eggs) to see if you can incubate anything at any given time.
There seems to be a fundamental misunderstanding of how hatching works, so let's clear that up. You can't just walk in place on a treadmill and expect to reap the rewards. The way it work is by way of GPS tracking, so you'll need to actually move about to get credit for distance. Yet, Niantic has worked in a failsafe of sorts that caps out how fast you're moving, so you can't drive down a highway at 60 mph and hatch eggs willy nilly.
Based on our tests, the cap is somewhere around 10-12 mph. That's roughly the average speed when riding a bike, provided that you aren't booking it or going downhill. Biking is not only a great way to exercise, but just like the core series, the optimal means of hatching eggs outside of cruising around a parking lot in your car.