The reality stars dressed up in patriotic attire for the run.
Later, they were seen packing on the PDA for cameras.
According to Us Weekly, the pair got pregnant after just one month of trying — and they couldn't contain their joy when they found out.
"Heidi was just standing there," 33-year-old Pratt told the publication of waking up to the happy news. "The look on her face, I can't even describe it. She was literally glowing. I thought she was about to say she made muffins or banana bread."
"Heidi said, 'I'm pregnant,'" the former The Hills star added. "I was like, 'Whoa, that's way more exciting than banana bread!'"
Montag is due October 19, Us reported. The 30-year-old revealed she was "overwhelmed" to discover that she was pregnant: "I started crying, and he embraced me."
"Heidi has wanted to be a mom since the day I met her," Pratt added. "She has planned for and thought about this."
We pay for juicy info! Do you have a story for RadarOnline.com? Email us at firstname.lastname@example.org, or call us at (866) ON-RADAR (667-2327) any time, day or night.