What you're bringing up is a severe logistical flaw in the storyline. "Brandywine Park" is 13.7 miles from "Sharp's Woods". Sharp's woods was where Erin's body was found, and is within the jurisdiction of the Easttown Police Dept so it is obviously in Easttown. Townships in that part of the country are usually a maximum of 2 or 3 miles border to border, maybe tops 5 miles in a rare case. So if Sharp's Woods was on the absolute border on one side and the Ross's house was right on the border on the other side (closest to Brandywine Park) that would leave 8.7 miles to get from the Ross's to Brandywine Park. So Ryan would have to ride his bike to the Carrol's to get the gun, ride 8.7 miles to Brandywine Park by midnight, then 8.7 miles back, plus going to the Carrol's shed to return the gun. That's at least 17.4 miles, which is a long way for a kid to ride a bike, especially in the middle of the night. It would take hours. As Mare said "No way Erin's going to ride her bike 13.7 miles to Brandywine Park".
Another problem is that before Erin's finger was found at Brandywine Park, her last phone ping indicated that it was 13.7 miles east of Sharp's Woods. And that's as the crow flies. 13.7 miles east of most places in Delaware County is going to put you in Philadelphia or the Delaware River. Or New Jersey. That is definitely not going to be a wooded area with less people, and it would involve riding through a lot of traffic, built-up congestion, and a number of towns. And the implication is that Brandywine Park is a rural, wooded area. The real Brandywine River is to the west and is the southern section of the western border of Delaware County. It is indeed wooded and relatively rural, although there is no Brandywine "Park". To get an idea of what it's like think "Andrew Wyeth". He lived on the Brandywine on the border with Chester County in Chadds Ford.
I grew up in the middle of Delaware County, and the locations in the show do not fit together in any way that resembles reality. However, for the sake of the story line, and the composite nature of the imaginary "Easttown", it is acceptable for me, and not something the vast majority of viewers could have any awareness of. But Ryan riding his bike to and from "Brandywine Park" in the geography as presented is completely implausible and ridiculous. Nevertheless, I otherwise thought it was a spectacular show with great acting. Plus, they did a great job on the accents. This is the first time that has been done for the Philly accent. It made me long for a Hoagie.