A primary point Paul makes is that we should "share in the suffering." At the time Paul wrote the letter, he was suffering in prison. Timothy was expected to be willing to suffer for the faith as well. In his words, Paul did not intend to use violence but rather to exhibit the attitude of a well-trained soldier. Military personnel are focused on their mission rather than on unrelated issues.
As he approaches his end, Paul reminds his spiritual son, Timothy, of the guiding principles in his teaching that are vital for sound spiritual development and growth. Timothy is encouraged to be strong in the grace embodied in Christ Jesus. He learned many nuggets of wisdom and teaching from his teacher, Paul. When faced with such difficulties, Timothy is encouraged to demonstrate discipline in the same way that a good soldier obeys his commanding officer.
Suffering is the badge of a disciplined soldier and faithful steward, for Christ Himself learned obedience through His own suffering. Paul provided an example of a man who endured suffering in the service of the gospel. Let us learn obedience through suffering, and let us bear hardship with patience and endurance in the place where God has placed us.

Dear Heavenly Father, it is a blessing to learn from Paul's example and teaching. It is my desire to be the person You want me to be, where You have placed me. Like the apostle Paul, may my love for the Lord Jesus move me to rejoice when I endure suffering. To the end of my life, may I be a good and faithful soldier and servant. I hope suffering will be my badge of honor for the sake of my Lord and Savior, Jesus Christ. 
In Jesus's name I pray. Amen.