I'm a 50 year old male. I can cruise along at 140-145 BPM for easily 30 minutes when running. On the trainer, even when pushing hard, I have trouble getting into the high 120s. This level of exertion isn't sustainable for more than 5-7 minutes. If I am doing longer set, same thing. Heart rate gets to about low to mid 120s. If I try to push harder to get it higher, I can't sustain the effort. As noted above, I don't have a power meter.
What is wrong with me?