Our sources tell us the "Pretty Little Liars" actress reached out to Colton soon after his split from ex-girlfriend Cassie Randolph ... and the two have been seeing each other a lot.

The ex-'Bachelor' went on a hike with Lucy Wednesday at Paseo Miramar Trail in Los Angeles' scenic Pacific Palisades neighborhood, and as you can see ... Colton and Lucy were pretty sweaty, close and without masks.

Our sources say Colton and Lucy have gone on a few of these "casual hike dates" over the last few weeks, so it seems their relationship has legs.

Lucy's had her eye on Colton for years too ... back in 2018, she said he would make a good 'Bachelor' because he's "wholesome," a "good guy" and "beautiful to watch." Oh, and we're also told they have mutual friends.

As for Colton being out without a face covering ... well, he's already survived his battle with COVID-19. True, outdoors is safer than indoors, but it's unclear just how much immunity -- if any -- people have who already contracted the virus.

BTW ... walks and hikes are kinda Colton's thing during the pandemic, only now he's transitioned from Cassie to Lucy.

Dude's definitely still a bachelor!!!