A word about earthly blessings
We have to avoid the temptation to over-spiritualize the Bible when we don't need to.
When the OT (Deuteronomic Covenant) talks about "live long in the land the LORD your God is giving you" (Ex 20:12) or even in the context of the NT (New Covenant) as when Paul (as a pastor) teaches the congregation how to live in light of the Ten Commandments "... which is the first commandment with a promise -- so that it may go well with you and that you may enjoy long life on the earth." (Eph 6:2b-3), it's obvious that Moses and Paul are referring to earthly blessing as God's reward for living righteously.
The Beatitudes (Matt 5:3-12) can then be more naturally understood as well, that those righteous people who did NOT receive earthly blessing before they die, will be rewarded in heaven (the new earth): "God blesses those who are humble for they will inherit the whole earth" (v. 5), "God blesses you when people mock you and persecute you and lie about you ... great reward awaits you in heaven. ..." (vv. 11-12).
Thus, Jesus, Paul, and the martyred 10 apostles (John was the only apostle who died a natural death), obviously did NOT enjoy the usual reward of long life on the earth. And they model for the subsequent martyrs and for us in the 21st century to be hopeful when God allow trials and sufferings to happen to us, just as Peter reminds us in his own sermon to his congregation: "... we have a priceless inheritance -- an inheritance that is kept in heaven for you ... There is wonderful joy ahead, even though you must endure many trials for a little while." (1 Pet 1:4, 6). Obviously Peter is referring to the Beatitudes as well !
Therefore, God's promises are NOT spiritualized. The righteous will enjoy a literal long bodily life, either on this earth, the new earth, or both (depending on God's plan for us). This reward is in addition to the blessings Jesus gave us (through his death):
- to overcome original sin when we have faith in Him
- to have a new heart that can obey God's commandments more readily
- to cast out demon that interferes with us
- to have more command over our depraved humanity (through the fruits of the Holy Spirit)
These blessings should be understood as separate from the "long life in the land", a reward of righteous living promised in Ex 20:12 and reiterated in Eph 6:3.
How several people are asked to postpone their earthly blessings
Out of love Jesus obeyed the suffering mission prophesied in Isa 53 where it includes an explicit reference that the usual earthly blessings will be denied to him (v. 8), namely: many descendants (v. 10b), long life (v. 10c), and honor (v. 12a). The passage clearly shows that it's God's plan for Him to suffer (v. 8c, v. 10a):
8 Unjustly condemned, he was led away. No one cared that he died without descendants, that his life was cut short in midstream. But he was struck down for the rebellion of my people. 9 ... 10 But it was the LORD's good plan to crush him and cause him grief. Yet when his life is made an offering for sin, he will have many descendants. He will enjoy a long life, and the LORD's good plan will prosper in his hands. 11 ... 12 I will give him the honors of a victorious soldier, because he exposed himself to death. ...
Like Jesus (a victorious soldier, Isa 53:12a), Paul also sees himself as a "soldier of Christ Jesus" (2 Tim 2:3). We are supposed to imitate Jesus (Matt 16:24, 1 Pet 2:21) and to imitate Paul (1 Cor 4:16, 1 Cor 11:1). We are then also eligible for the earthly reward denied us, if God has a plan to make us suffer out of love for God and for our neighbors.
If instead God's plan for us does NOT leave us "cut short in midstream", we can then expect earthly blessings and we should use them in the manner that God clearly taught the Israelites in the OT:
- honor God with those blessings by giving thanks to Him
- raise Godly descendants
- use the earthly blessings to help the weak, the persecuted, and the victims of injustice ("the widows, the orphans, and the unjustly imprisoned").
How should we interpret this verse [Ex 20:12] in terms of its applicability?
It still applies to us under the New Covenant, it has NOT been abrogated. The OT promised land of Canaan is a prefiguration for prosperous earthly life for us, whether it is in America, Canada, China, England, etc.
According to Paul [Eph 6:3], did Jesus enjoy long life on the earth?
No, because Jesus accepted God's mission to suffer out of love, and accepted the postponement of the reward. He would have relied on Isa 53 where the same passage that prophesied His mission also prophesied how He will receive His postponed earthly reward as well. We, as imitators of Christ, can expect the same "suffering -> postponed reward" as well, clearly taught in the NT, such as in the Beatitudes and in Peter's sermon: "So when your faith remains strong through many trials, it will bring you much praise and glory and honor on the day when Jesus Christ is revealed to the whole world." (1 Pet 1:7b).