I use a Cateye with Cadence combined with a Polar.
Long hours on a trainer are more interesting if you see cause and effect.
Specifically go up with cadence but hold speed to a constant (gearing down) and its effect on HR. Next study effect of frequent hydration say every 15 minutes.

On a recent trip to Florida I trained on one of their flat trails. They call it undulating.
The headwind from the south was substantial. I did 17 MPH at 90 RPM. I glanced at my Polar and it was up to over 140 which is high for me.
I cannot imagine not having these tools.