Did jack today. Just walked on the boardwalk.
Some thoughts of mine...
Easy walking is good for recovery. Low stress.
I always had best results taking a rest day between workouts. This was the tried and true method for nattys in the old days.
Once steroids came along the game changed and people found they could train everyday because the drugs put them in a constant state of anabolism.
Youth may also allow you to do this because your hormone levels are at their peak. Even in my youth I still found a day off to work best. I wrestled in high school and was always in an over-trained state and that was really not good. It sucked then and it sucks now.
Running 3 miles would not be a rest day if I was also weight training the next day or the same day. If I ran 3 miles then I'd have to train upper body the next day but not legs. Even so, the running, if intensive, would probably affect my recovery for my next weight workout. If it was relaxed it most likely would not. Easy walking, light jogging would be ok. What would make it intensive? Competitiveness, trying to improve my time, trying to be faster, etc. Pushing yourself.
Dorian Yates advised not to do cardio on weight workout days as it would impede muscle recovery. He did light cardio on off days. The key here is non-stressful cardio.
Intensive weight training affects your nervous system and you need recovery time. If you keep draining your body and nervous system and don't allow it to recover you eventually end up in an over-trained state which can result in feeling like crap both physically and mentally.
If you look forward to training you are likely not over-training. If you have to force yourself, or dread having to train you likely are. Training refers to both weights and running in your case. Training encompasses all of it.
The body and mind are interconnected.