RE: heart rate???
At my age (19), doing 65% MHR doesn't feel like any work either - but that's why doing long-duration fat loss cardio has to be long in duration. By the end, if I'm lucky, I might break a sweat... but hardly anything compared to HIIT training. It still works though, even though it doesn't feel like it. If you do 160 bpm, which is nearing your max, you will end up eating your muscle for energy if you exercise for long durations. This is why there is a 'fat loss zone' and a 'cardiovascular zone'. One is targeted for fat loss, the other is for fitness.