From the pure technology point of view not too hard, but there are business and logistics challenges. Apple is highly optimised to hold minimum stock near the customer and use a lot of just-in-time delivery, but this depends on a high product value relative to weight and volume of the parcel. Treadmills and exercise bikes are only really cost effective to ship by ocean.
It's also doubtful that the addressable market of an expensive treadmill or exercise product is large enough to be worth going after for Apple (even in the bumper year 2021 Peloton turnover was $4bil, not a viable business unit for Apple).
They would probably make something like FitnessKit similar to HomeKit, so that your iPhone can adjust settings on your fitness device. Fitness device makers can then make apps to give you exercise programs and track your progress. It might already exist. I don't know.
It's also doubtful that the addressable market of an expensive treadmill or exercise product is large enough to be worth going after for Apple (even in the bumper year 2021 Peloton turnover was $4bil, not a viable business unit for Apple).