My point was more that you shouldn't delay going to train because you're out of shape, not that you shouldn't also get in shape outside. I still recommend you start training as soon as you can afford it, if that's your goal, but if you're looking to get into shape at home, there are a couple of things that I'd recommend.
1. Jump rope. Can be really good cardio, you can choose your own pace, and the number of reps, and compare those reps day by day easier than running. Also doesn't take a whole lot of space and you can do it in the comfort of your own backyard (or inside if you've got the ceiling for it, or park if you don't have a backyard).
2. For less endurance focus but general strength training, check out convict conditioning. There's a book with the exercises, I'm sure you could find a free pdf of it with very little digging, but as a moderator, I do not recommend that. It's useful for general strength training exercises, again has ways to measure your progress and body progression as you get stronger (also has options for those who are not in shape at all to start at stage 0), and gets most of the needed muscle groups.
3. Couch to 5k app. It's an app (available on android and iPhone) intended to get anyone from not exercising at all to be able to run a 5k within 9 weeks. Works for those that stick with it, barring medical issues they may have.
4. If you're a nerd like me, check out
Workout Database. It provides workouts that would 'fit' specific superheroes. For instance, the blob is an x-men villain that is pretty fat but strong, like the original strongmen. His workout involves 4days per week of pretty much just strongman workouts: deadlifts, pushups, things like that, 2 rest days, and 1 "active" day doing a non-specific sporting activity, which is useful if you want to get strong, but not to just get in fighting-shape on its own. Meanwhile, the green arrow has a workout much more appropriate to martial arts, with calisthenics, endurance training, and muscle-building that gets all the main muscle groups. The only thing is that can be a tough workout for a beginner. They've also got diet options, but last I checked you've got to pay for that.
I've done a few of these because I get bored of workouts if they get too monotonous, and would recommend the captain America one if you've got the materials needed, and the echizen ryoma one if you don't. The echizen ryoma one is tennis based but does a pretty good job for other purposes as well.
5. If you've got a school or stadium you can go into free that has bleachers, running up and down stairs is an amazing way to build up cardio. If you don't, running up and down a hill (or doing things like a bear walk up the hill and sprinting down) is a good alternative. Then switch off between sprints and walking once you're done.
6. Lastly, look into HIIT. These are tougher workouts, involving a lot of intense aerobic activity followed by resting and repeating. These can be tough to stay motivated for, but are good at getting you in-shape while requiring less time during the day, and less days to improve your fitness level as well. Here's a link to a list of different-timed HIIT workouts I found, but unlike everything else on this list I have not tried this specific list so can't give you any recommendations.