No matter what branch of service you choose, you are going to have to get your cardio fitness levels up. To do this we would recommend some basic interval training where you run a set distance and then walk a set distance while repeating the process for around an hour, three to four times a week. You will quickly see results and increases in both the speed and the distance that you are able to run for.
In addition to this, pretty much every branch of the military will also require you to do sit-ups and press-ups as a part of their fitness test for entry. We would recommend that you simply set a two-minute timer and smash out as many press ups as you can within two minutes followed by a little break. Repeat this two more times and track how many press-ups you can do, keep doing this every single day and track your progress, do the exact same method for your situps.
The final thing that you will probably have to pass when joining either an Army or Marine unit will be the chin up or pull up. In our opinion, this is without a doubt the hardest one to get good as as your current body weight will come into play as you have to lift your self. The lighter weight you are the easier it will be for you to be able to smash out your chin ups and pull ups but try something similar to the press ups and sit ups that we covered earlier to see how you get on.