Since 1979, Doug Laufer has run least one marathon a year. He has also completed at least marathon in every state. But, there is one race that impacted more than any other.

"Not only heard but felt what turned out to be a bomb blast, we didn't know that at the time," Laufer said.

In 2013, Laufer finished the Boston Marathon exactly one minute after the first bomb went off.

"I do think about what if I had been right there when the bomb went off?" Laufer said. "What if I had been one of those people, thousands of people who didn't get to finish the race that were stopped miles from the finish and didn't know what was going on?"

Three people were killed four years ago with hundreds more suffering injuries. Sixteen people lost limbs when two bombs exploded near the end of the 26.2-mile course.

"I just heard the second one, I felt and heard the first one," Laufer said.

He is a professor of accounting at Metropolitan State University of Denver. Saturday morning. he left for Boston for the first time since the bombings. Laufer will not be running. He will be a coach and mentor for his girfriend, Betty Connor, who is running the Boston Marathon for the first time on Monday.

"This year, I think it'll be emotional and it'll be more emotional to be on the starting line again," Laufer said.

Over the past couple years, Laufer has been dealing with health issues including a hip replacement. He hopes to be in shape to qualify for the Boston Marathon next year.

"Don't plan the bombing to be my last marathon," Laufer said. "You don't want to let the bad guys win."

Though, he admits, going back is not easy and talking about the attack has been tough over the last four years.

"Trying to understand why stuff like that happens, why people try to steal life's positives, life's enjoyments," Laufer said.