If you fail to make it to 300 steps without having to stop through fatigue, strength-endurance may be something you need to work on for your calf muscles.
If you’re struggling to get your low back flat to the wall, it would be worth you working on hip flexor mobility again.
If it gets easier with a wider stance, you may be tight through tensor fascia latae on one of both hips. This would be worth working on, in such cases.
Big Toe Extension Test
If you notice a marked lack of big toe extension on one or both sides, my suggestion would be for you to consult a physio/podiatrist to determine the source of this stiffness. Functional vs structural limitations here will require different treatment approaches.