What does Romans 12:1 have to do with giving up something for a certain period of time then going back to it when that time is passed.
Romans 12:1 I beseech you therefore, brethren, by the mercies of God, that ye present your bodies a living sacrifice, holy, acceptable unto God,
which is your reasonable service.
https://www.gotquestions.org/what-is-Lent.html
The length of Lent was established in the 4th century as 40 days beginning from Ash Wednesday to Easter. It's only for the purpose of self discipline. It's simylar to how people in the OT fasted and repented in sackcloth and ashes, Esther 4:1-3; Jeremiah 6:26; Daniel 9:3.
However, over the centuries Lenten observances have developed a much more "sacramental" value. Many Catholics believe that giving something up for Lent is a way to attain God’s blessing. But the Bible teaches that grace cannot be earned; grace is “the gift of righteousness” (Romans 5:17). Also, Jesus taught that fasting should be done discreetly: “When you fast, do not look somber as the hypocrites do, for they disfigure their faces to show men they are fasting. I tell you the truth, they have received their reward in full. But when you fast, put oil on your head and wash your face, so that it will not be obvious to men that you are fasting, but only to your Father, who is unseen” (Matthew 6:16-18). Jesus’ command to “wash your face” seems to conflict with the practice of rubbing ashes on one’s face on Ash Wednesday.
Fasting can be a good thing, and God is pleased when we repent of sinful habits. There’s absolutely nothing wrong with setting aside some time to focus on Jesus' death and resurrection. However, repenting of sin is something we should be doing every day of the year, not just for the 46 days of Lent.
If a Christian wishes to observe Lent, he is free to do so. The key is to focus on repenting of sin and consecrating oneself to God. Lent should not be a time of boasting of one’s sacrifice or trying to earn God's favor or increasing His love. God’s love for us could not be any greater than it already is.