Treadmills are predictable. Trails have roots and rocks. Only one of them trains real balance.

Controlled conditions build one kind of fitness and messy conditions build another. Everything predictable lets you improve within a narrow band, which is useful right up until something unexpected happens. Handling uneven ground, changed plans and problems nobody warned you about is a separate skill, and it develops only where the ground is genuinely uneven. Comfort is not the enemy; it simply cannot teach that part.

Put yourself somewhere slightly unpredictable this month. A harder class, a new position, a job that is not scripted. Expect to look worse at it for a while and stay in anyway.