SB 4.23.4

tatrāpy adābhya-niyamo
 vaikhānasa-susammate
ārabdha ugra-tapasi
 yathā sva-vijaye purā
Synonyms: 
tatra — there; api — also; adābhya — severe; niyamaḥ — austerities; vaikhānasa — rules and regulations of retired life; su-sammate — perfectly recognized; ārabdhaḥ — beginning; ugra — severe; tapasi — austerity; yathā — as much as; sva-vijaye — in conquering the world; purā — formerly.
Translation: 
After retiring from family life, Mahārāja Pṛthu strictly followed the regulations of retired life and underwent severe austerities in the forest. He engaged in these activities as seriously as he had formerly engaged in leading the government and conquering everyone.
Purport: 

As it is necessary for one to become very active in family life, similarly, after retirement from family life, it is necessary to control the mind and senses. This is possible when one engages himself fully in the devotional service of the Lord. Actually the whole purpose of the Vedic system, the Vedic social order, is to enable one to ultimately return home, back to Godhead. The gṛhastha-āśrama is a sort of concession combining sense gratification with a regulative life. It is to enable one to easily retire in the middle of life and engage fully in austerities in order to transcend material sense gratification once and for all. Therefore in the vānaprastha stage of life, tapasya, or austerity, is strongly recommended. Mahārāja Pṛthu followed exactly all the rules of vānaprastha life, which is technically known as vaikhānasa-āśrama. The word vaikhānasa-susammate is significant because in vānaprastha life the regulative principles are also to be strictly followed. In other words, Mahārāja Pṛthu was an ideal character in every sphere of life. Mahājano yena gataḥ sa panthāḥ: one should follow in the footsteps of great personalities. Thus by following the exemplary character of Mahārāja Pṛthu, one can become perfect in all respects while living this life or while retiring from active life. Thus after giving up this body, one can become liberated and go back to Godhead.