You weren't trying hard enough.
+1. Get some Spinervals videos, specifically interval-based ones, and drive that heart rate up. If you do it right, you'll be destroyed and cross-eyed after 45 minutes (including proper warm-up).
HRM is an absolute must if you're not doing all-out intervals, as I find it much harder to drive up the heart rate for tempo or medium-effort workouts on the trainer.
"The song of a bird…We used to ask Ennesson to do bird calls. He could do them. How he could do them, and when he perished, along with him went all those birds…"-Return from the Stars, Stanislaw Lem
"We just walk around, and sometimes we go out and dance, and then we listen to the environment."-Ralf Hutter, Kraftwerk