As a reasonably fit runner and cyclist, my experience is that 120 BPM is sustainable for a workout. I can certainly get to 150+, but unlike when running a 5K or cycling a time trial, I don’t find it sustainable. You’re using large muscle groups to do the work, and those muscles just aren’t moving that quickly. Think lifting versus cycling. You’ll get a higher BPM workout if you swim the water versus paddling it!
Note, I got into a flow state yesterday in my kayak. First time in flow state in a long time (sidelined by minor injury) and I think the first time in my kayak. Felt fantastic. For those who don’t know what “flow state” is, it’s kinda like being in the groove…all of a sudden everything gets easier and you’re just completely present to the task at hand. Runners, cyclists and swimmers aspire to finding it.