One suggestion would be to park at millvale riverfront park, take the trail towards the casino. You’ll then take the west end bridge where you’ll run along west Carson. You’ll make a left into the station square incline parking lot area where you’ll be able to access the GAP trail & can continue on for your desired mileage. Or skip all that, park at the station sq lot and run the gap trail from station square.

I personally avoid the trails unless I’m doing a speed workout bc I get bored on them. I prefer to actually run the marathon course itself to get used to the hills etc but I understand not everyone enjoys running on sidewalks.