Hello, my name is George Bouris.
I have more than 20 years of experience on designing and programming electronic systems.
I think that a desktop application is a good starting point, with the other kinds to follow.
Regarding the sensor, most of the stationary bikes have built-in such a sensor, maybe we can get the pedaling status from this sensor. If you don't want to mess with existing electronics, a tiny magnet/Hall effect sensor can be used. Finally, it can be a wearable motion sensor. Of course, other options exist.
An Arduino compatible, but much smaller in size and cheaper, will be used for the capture of the sensor's signal and transmission to the main application.
If it will be permanently installed on the bike, the sensor and the main device will be wired. Otherwise bluetooth communication will be used.
This is the general idea, we have a lot more details to discuss.