Today was my first day back in the gym since getting my back worked on Monday. I intended to go for one hour easy-peasy on the recumbent and did; but then got hooked on the reno show about a 1911 house and ended up going two hours and tempted to do a third. No worries, I stopped.
I was at about 70-75% HR for that time.
It's no secret I like using a telemetry strap (chest strap) for transmitting HR info but I found an
article that points out some other benefits of trying it for the curious.