​​​​​​Agree. If it's going to work at all, it'll probably be by pulling the speed magnet off the wheel.

You may want to consider a power meter. It's an objective way to measure exercise intensity that works even for people with abnormal HR response. Plus a lot of them report cadence, and they give you data that's much more valuable than anything else you can measure on a bike. I wouldn't normally recommend one, but in this case it's maybe worth thinking about.

Don't push yourself too hard!