One of my biggest pet peeves around trail running is the stigma that if you're not ONLY on a dirt/rock trail, you're not a trail runner.
If you run on a trail whether it's weekly, monthly, etc., you're a trail runner.
Just as any other sport or activity, if you do X, you are a X-er.
If you ski, you're a skier. If you road cycle, you are a road cyclist. If you climb, you are a climber. Someone who snowboards twice a year will still call themselves a snowboarder.
So if you can't get to a trail as frequently, whether it's because you're busier than usual, the weather isn't cooperating, or you simply don't want to make the trek to a trail, do the following instead.
For your run portion, hop on a treadmill or take it to your neighborhood sidewalk. Once you've completed your run, grab your weighted pack and finish out with some step-ups. I like to use water or other drink bottles, camping gear, heavier books, or dumbbells in my pack to add weight. If adding weight is too much to start, do body weight step-ups.
This will be one of the closest ways to emulate a trail run. No, it's not dirt and rocks, but it's pavement or a treadmill belt and a step to get that run in with "elevation gain".
This is also a great way to learn how to keep moving on tired legs. After your run, your legs may feel a little cashed depending on your intensity and duration. Finishing off your workout with step-ups will really take your intensity and capacity to the next level.