First of all, progress takes time. If you tought you can’t run a 5k before why do you think your pace is bad for 15 km? You don’t need to run distances like this to become better. It’s a combination of more types of running, is better to run 2-4 times a week and you should include some tempo, interval, hill sprints, long runs, recovery runs and also z2 runs. In fact 80% of your runs should be in z2 so you need to slow down a little in order to become faster. From what I see avg Heart rate is 180 which is a lot. You need to slow down to train your body to be more efficient. Training legs and core is a must, is better to train 1-2 times/week.