If you find 0-5k a bit easy, maybe do your own plan along the lines of 3 runs a week all around 5k one easy, one medium,can done faster. Building up the distance on one of the runs half a km a week or similar? and having a drop back / rest week every 4th week.
i find having a target a good way of staying motivated, and as for injuries, if it's always the same area then that is pointi to a weakness that you need to strengthen which can be done with cross training, say core work, or targetting specific muscle groups.
you don't need to join a gym, you can use kettlebell weights, and resistance bands, and do training at home.
Just remember, with the diet as well that's pretty much done to simple physics, and you need to concentrate hard on limiting the amount of calories you take in to ensure it's less than you burn.