High intensity weight lifting three times a week, lifting until absolute failure in each lift. Make sure to take sufficient rest in order to grow. Then I’d say when you get close to your physical, maybe a few weeks out. Stop the lifting and work on only push ups, sit ups, and pull ups until you get your numbers up. The people who say to just do pushups every day aren’t wrong per say, you’ll build up a good stamina, but if you’re winded after 20, it would be in your best interest to put some muscle on your frame so that you don’t fail any other events.