Hi Ariel, given that no one has responded to you, I thought I’d give my two cents. Disclaimer: I am an amateur and all of my info is from reading the books and this forum. Directly answering your question, I think doing a long trail run with a little bit of elevation is no problem if you are keeping your HR in check.
A longer answer: I think a lot of the information here is to help people learn how to approach and think about these questions with broad principles, since everyone is coming from a different background with different goals and the answer will be different.
So in that vein, I would ask first: what is your goal? Are you trying to be an elite runner competing in pro road races? If so, then you want your training to be as specific to that event as possible and trail running might be less than optimal. Or are you just trying to improve your fitness for some more modest recreational goals like finishing a marathon or placing in your age group at a local race? If so, varying your training is probably going to not only be fine for your goals, but likely good to stay consistent by keeping it interesting and motivating and fun.
On a more specific scale on how to modify what’s on your training plan: The question you want to ask yourself is what is the goal of the workout? Are you trying to provide an aerobic stimulus or are you strength training or is it a muscular endurance session? In this case, since it sounds like it is the long run for the week, the goal is likely building aerobic fitness. Since you already noted you would be keeping your HR in check and focusing on duration and not some kind of mileage goal, then it brings me to my initial answer that swapping for a trail run with a little bit of elevation will likely meet the goal of your workout.
