If you did not pray at all, not  single prayer, the time is missed and gone as you were not a Muslim at the time. You can offer nawafil prayers and many good deeds and observe your prayers and fasting on time.

I never prayed or fasted back in may jahiliya days. Since then alhamdu Allah I have deeply repented and regret and never have intentions to go back. I estimate I missed around 10 years of ibada what do I have to do ??