The stride length doesn't matter really.
You should start lifting and doing hills. To run 47.6 you'd probably need 22.5 or better, so focus on the 800. Your 1500m time is not good, but that fact that you did one shows that you probably feel more like a mid distance runner than a sprinter. That actually matters.
With your times in 400 and 800, you should be around 4:05 for 1500, which is not bad. The workouts you are doing are definitely 800 workouts. With proper endurance training, you could run 48.5, 1:50, 3:50. That is very good.
I suggest long intervals at current 1500m pace (3x1000m in 2:50, 6min recovery), and tempo runs of 4km at 3:30/km. These will get faster as your endurance improves. Ultimately, you should be able to do 3x1000 in 2:40 and the 4km tempo at 3:10/km. (I used metric since it seems you are from Europe)
Good luck!