In plain words
For if I ever ceased to engage tirelessly in action, people would follow my path in every way, Partha.
Word for word
For if I did not ever continue unwearied in action, people would follow my path in every way, O Pārtha.
The commentators
Bhagavadgītābhāṣyam· bhāṣya
यदि हि पुनः अहं न वर्तेय जातु कदाचित् कर्मणि अतन्द्रितः अनलसः सन् मम श्रेष्ठस्य सतः वर्त्म मार्गम् अनुवर्तन्ते मनुष्याः हे पार्थ, सर्वशः सर्वप्रकारैः ॥ २३ ॥
In plain words
Suppose, on the other hand, that I never engaged in action — not once, not at any time. Atandritaḥ means never slack, never idle. My path, the road of one who is highest, is the road people walk, Pārtha. Sarvaśaḥ: in every possible way.
Close to the Sanskrit
If indeed, on the other hand, I should not ever — at any time whatever — engage in action, being atandritaḥ, that is, not lazy: my path, the road of me who am the highest, men follow, O Pārtha, sarvaśaḥ, that is, by every kind of means.