As mentioned previously, it's pretty easy for new runners to think running has to be hard and so will often run at a higher perceived effort level than is actually beneficial (without suffering too many downsides as runs are short/infrequent). Your cadence also seems low for your pace suggesting you may be taking longer strides than is "best" (although by no means is it guaranteed, i think Mo Farah has a cadence of 160 and look at how quick he is!)
Your HR starts off lowish and jumps quite early on, which corresponds with a jump in pace and then never recovers, presumably due to lack of a sufficient base level of endurance fitness.
What i'd suggest would be to have a play with the data screens on your watch, create one with just HR and Time and then next time you run just try and keep HR under 160 as an initial goal (ideally under 150), without your watch telling you your pace you'll be less conscious of trying to run at whatever pace you think you should run at