When you're starting a new habit (or even sticking to an old one), the more things that are in your way, the more opportunities you have to break it. Rather than trying to start the habit in its best form possible, focus on removing obstacles first.
As life improvement blog Riskology points out, obstacles are the enemy of the habit. You want to start working out, but that can mean buying working out clothes (which you don't have money for) and finding a gym (which you don't have time for). Except, it doesn't have to. Rather than trying to get the most optimised version of the habit right off the bat, take the minimum amount of steps necessary to get started. Once it's a routine, you can start improving the habit as you need to:
I'm successful with my running habit (and wildly unsuccessful with others) because I've decided that the only thing that will stop me from getting my runs in (repetition) and improving my streak (momentum) is illness. Not waiting to get a good deal on running gear. Not feeling too busy. Not bad weather. None of these potential failure points are a part of the habit-maintenance process for me. If I'm alive and feeling at least reasonably well, I'm running on a pre-determined schedule.
This might mean that your habits aren't revolutionary changes right off the bat. If you do 10 pushups every day on your lunch break, that alone probably isn't going to get you in shape. It can, however, get you used to thinking about working out and, once you've got that down, you can start adding exercises, or building on your routine. The important thing is momentum.