You can get a dual band speed/cadence sensor, like the Wahoo BlueSC, and connect it to a cycling app on your phone that supports Bluetooth LE sensors. A good free app for what you want to do is the Wahoo Fitness app. Most other cycling apps charge for the ability to connect to sensors. Since your rides will always be indoors, you really don't care about GPS tracking and really don't need to pay for that service, either.