Start small and build up a routine. Try to use the stationary bike (or walk outside) for 15-20 minutes a day, either in the morning or in the late afternoon. Intensity doesn't matter so much as getting your schedule and routine adjusted does. Do this every single day for a week- set reminders, give yourself an incentive for completing it, etc.

Next week, increase the time you spend. 5 minutes, 10 minutes, maybe even more if you feel like your body can handle it. The following week, increase either duration or intensity. Bike more, bike at a higher speed, walk more, shift the walk into a brisk walk or a light jog. I think once you get into the habit of exercising then you can start to follow a specific plan and begin focusing on muscle groups to strengthen!

As a kid, you maintain a sense of bewilderment for the world and what you can do simply because you don't know what limits there are. Once you become an adult you become burdened by the weight of responsibility and expectations and that bewilderment becomes dulled. I do think it can inevitable that we limit what's possible but we also legitimately have less time and resources to do the things we want as an adult than we do as a child. You can have a lot of free time (at least, for those who are fortunate) to think, dream, and enjoy youth.

I'm 27. I bought a drawing pad 2 years ago and practiced digital art for about a year before I lost interest/got caught up in other things. I really enjoyed it, and still draw on occasion, but I have to study for a professional engineering exam and plan out a trip to Japan this year. I work solely for the purpose of making a stable income and spend my spare time doing things I actually am more passionate about- most of the reason for this is because making money doing things I love (I.E. Art, Games, Music, Etc.) would be extremely risky and would take an insane amount of time and effort to match the income from a career job in engineering. It's possible to quit your job and pursue your real passions, there's people everyday who do it. But how many of those people actually succeed? How many of those people are actually more satisfied than if they just kept their hobbies as hobbies? I'm not sure. And fear of uncertainty is certainly a powerful factor in preventing me and probably most others from making that choice. Do we have to go from dreamers to realist? No, but reality is accepting that you don't want to take a big risk simply due to uncertainty.