Haren pitched two innings, allowing the one run on two hits while fanning three.
"It felt good, especially as I was going along. It was kind of a circus to start," Haren said. "The Marlins really came out aggressive. … I was a little out of sorts in the beginning. I fell behind guys. It's not my game to be [behind] 1-0 or 3-0. As the game started going, I was throwing more first-pitch strikes and getting better looks."
Haren, who has won 119 games over 10 years in the Major Leagues, was not afraid to admit that he had butterflies in his stomach.
"Every time I've taken the mound in my career, there are nerves," Haren said. "I was nervous waking up today. If you are not nervous, you shouldn't even do it anymore. But this is what gets us going -- competing. It starts today."
Haren threw mostly fastballs and cutters, and he is looking to throw more fastballs inside on right-handed hitters.
"My game is all command, control -- keeping guys off balance, in and out," Haren said. "That's got to be a part of my game this year. I started doing it in September last year. I was getting beat over the plate too much. I had a lot more success at the end of the year. I came in and I'm dedicated to working inside of the plate this year."
After signing a one-year, $13 million contract with the Nationals last December, Haren joined a formidable rotation that includes Stephen Strasburg and Gio Gonzalez. Haren says he is feeling comfortable in his new surroundings.
"Any time coming into a new clubhouse, it's difficult," Haren said. "You are basically thrown in with a bunch of guys and you have to become friends with them. And I'm more of an introverted personality, so it's a little more difficult for me. I'm starting to get to know the guys. Guys are opening up to me, and I'm opening up to them.
"It's going to be a journey, it's going to be a fun year. I've heard nothing but good things from guys that I know. Adam LaRoche and [Chad] Tracy. Those guys have told me that the guys are great, and I can't wait to go out and go to war with these guys."