Not 1 little bit. Not even a small amount, not at all.
The odds are so minor, that even if you were at a park and it got attacked, you would most likely not be in the attack, if it even happened, which its self if highly unlikely.
The reality is that 1) it can happen any time any place, and 2) the US is doing a much better job than you know at controlling access to the parts required to put together an attack. As soon as they get word of someone looking to put a device together, they are all over it. Somewhat recently a group tried to blow up a bridge near me. They ended up being surrounded by undercover agents, and informants, and they had no idea. They went so far as too "sell" them parts that they though were real, and made the arrests when they were on their way to plant it. Big news here locally, but didn't get much national attention.