Need help in the gym.

Lu

Ok so I’m currently going through the selection process to join the police force and I am due to do my physical in March 2018, I need some tips on getting my fitness up as I have to do a 2.4 km run in 10 minuets and I need to be able to do 20 push ups but due to past injuries my back is very weak! So any help would be great! Thanks in advance