How can I become more physically fit?

I'm not worried about my weight or anything, but I want to become more physically fit. I don't want to tire easily. I want to be able to do more push-ups, sit-ups, leg-raises, squats, etc. Especially the Push-Ups. The only thing I don't understand is, how do you become more physically fit? Do you workout until you're literally about to pass out, and you have more durability over time, or what? I take Karate 3-4x a week, and it's definitely tiring. They don't fool around with working out!!! I'm starting to drink more water as well.

you really just have to practice a lot and no you dont work out until you pass out, just until you feel like you have worked out, that sounds stupid but whateves lol the more you workout the longer it will take for you to feel the pain from the workout. oh yeah waters a good idea. and for a good workout try the eight minute series, look it up on youtube theres a eight minute arms one.

Oh I miss karate, what do you take? I took shotokan for 6 year. I also did a little judo, aikido, and taikwondo. Anyway, as for push-ups squats etc you just have to practice. Those are muscle exercises and you need to build more muscle to do more.

If you want more endurance then you can do wind sprints. The really help build your endurance. You sprint as fast as you can for 30 seconds then rest for 60 seconds then do it again. Do it as many times as you can repeating that. Start off easy maybe around 4. Do it 3 times a week and every week add one more.