But minutes later that joy turned to horror for the New York couple when a pair of bombs exploded Monday afternoon near the finish line less than 1,000 feet from where they stood.
"I just crossed the finish line with my wife," Arnstein, who works in e-commerce, told DNAinfo.com New York. "We were celebrating, very happy. Everybody around was just excited.
"And the bombs went off."
Boylston Street was packed with hundreds of runners and spectators when the two devices went off around 2:50 p.m., killing three and injuring scores more in a horrifying scene that sent bystanders dashing for their lives.
DNAinfo.com New York spoke to some New Yorkers who were there who shared their terrifying accounts.
"The first one went off and everything shook," said Arnstein, who normally finishes marathons much more quickly but wanted to keep pace with his wife this time.
"There was a big cloud of smoke... a mushroom cloud," he said.
"You could feel the shockwave going through your body."
A few seconds later, the second bomb exploded, and any doubts bystanders had as to what was going on were erased.
"It was like 9/11," said Arnstein, who finished just after 2:37 p.m. with a time of just more than 4 hours. "When that second bomb went off everybody panicked."
"People were like, when is the next one going off?"
The explosion rocked the street, sending a shockwave through the buildings in the tight corridor.
The thunderous blasts, caught on video, sent the crowd, which was exuberant moments before, scrambling for safety.
"Everyone was running in every direction," Arnstein said. "It was just chaos.
"People were screaming and people started to cry. "
Arnstein, who has competed in 18 Boston marathons and was the fifth American finisher in the New York City Marathon in 2011, grabbed his wife's hand and took off for the safety of a side street.
"Everybody was running to get out of the area," he said. "It was every man for himself."
Luckily, both escaped unharmed.
"We just missed getting hit with this bomb," he said.
It was a similarly chaotic escape for Scott Weiner, one of "The Running Rabbis," who used to serve a congregation in Washington Heights and is now the senior rabbi of Temple Israel of New Rochelle.
Weiner, 37, said he had finished running the race — his 10th marathon and his first Boston run — and was heading to meet his wife and two young daughters in the VIP spectator section when he heard the first explosion.
"They sounded like bombs," Weiner told DNAinfo. "It was really loud. Add then when the second one came a few seconds later, there was no doubt.
“I didn’t think for a second that it was an accident," he said.
Weiner said he immediately began pushing through the fleeing crowd, trying to reach his wife and little girls, aged 3 and 6, whom he said were directly across the street from the first explosion.
“Lots of people starting coming towards me, away from the finish line area,” he said. “I pushed my way in because I knew my family was down there.”
While he didn't have a clear view of the explosions, he said the scene was surreal.
"[There were] people covered in blood, being pulled away on anything they could find: golf carts... gurneys from ambulances," he said. "It was really a crazy, hectic scene.”
“All I saw were the injured and the casualties and the mayhem around it," he said, describing the injuries as "severe."
Thankfully, he was able to reunite with his family at a friend's hotel room two blocks away.
"It was a little emotional," he said. "Especially just the four of us being together."
He said they were shaken, but alive and unhurt.
“My girls are young. They asked, 'Why, why was there an explosion?'" he said.
His wife, he said, described the explosions as “the loudest noise she's ever heard in her life."
Despite the nightmare, Weiner said he had no intention of abandoning "The Running Rabbis," who have been raising money for the Boston's Dana-Farber Cancer Institute.
Would he consider running the marathon again? "For sure," he said.
Brendan Byrne, who works in finance in Midtown, also had a close call.
Bryne, 29, said he was about 500 yards from the finish line, about five minutes after finishing the race, when the bombs exploded.
"It was just a tremendously loud explosion. And you could see the smoke right away," he said. Within 15 seconds, police officers were sprinting to the scene.
"My parents were both there. They were both standing where the flags were," he said, referring to the flags lining the spectator rows, directly adjacent to the explosion.
Fortunately, they had left shortly after he crossed the finish line.
"Lucky. That’s sort of the feeling right now," he said of their escape. "It's pretty scary stuff."