Yeah, check around to see what type of Boot Camp Fitness Tests your military does.

The US Army is pretty simple: 2 mintues of Situps and Pushups,then a timed 2 mile (3.2km) run. The number of situps and pushups you need to "pass" are based on your age/gender(probably still) and any "profile status" and the time you need to do the 2 mile (3.2km) run is is based on the same.

So if you can jog a few times a week and get through 2 miles (3.2km) in like 15 minutes you're probably doing pretty good. I actually had pretty good results with doing a few 2~5 mile ruck marches once or twice a week as well. Get a decent backpack that fits comfortably and add weight to it over time. There's also something called "interval training", where you alternate sprinting and jogging either a distance or a time. So if you were on a track or a roadway with some way of gauging distance, you'd jog between two points and then sprint between the next two points and then jog between the next to points. Or if you had a stop watch, you'd sprint for 15 seconds and jog 15~30 as a "break" before sprinting for another 15 seconds. Increase the time/distance as you get better at it.

Pushups are pretty straight forward, you're working on being able to keep your back/neck/hips in straight line while lowering yourself to the ground so that your upper arms are parallel to the ground. You can just drop and knock out 5, 10 or 15 constantly throughout the day and wind up doing 100 pushups total in a day no sweat. Moving your hands into different positions relative to your chest works different muscles to give variety.

Sit ups can take a bit more effort. ( At least for me, my back has never been in great shape) Find some demonstrations of abdominal and lower back/hip workouts to strengthen your core and you will have a MUCH easier/comfortable time doing sit ups. You can probably do these in the same way that you can do the pushups, short sets with low repetitions constantly throughout the day with occasional attempts to see how many you can do all at once. SUPER IMPORTANT: get a foam mat to lay on while doing sit ups, it helps keep the pressure off your tailbone.

Probably throw in a few pull ups/chin ups into whatever routine you're trying to maintain and you will be doing alright.