I experienced a bunch of side-effects that pretty much all disappeared after the first ten days. I read online that they cleared up for most people, sometimes taking a month. The only side effect I have now is an increased heart rate approximately 15bpm but it's still considered in the normal range and exercising helps. I feel slightly less able to focus after a couple of months of taking it, but I'm definitely a lot more productive than I was before so it might just be my perception of the effectiveness. Emotional regulation was my biggest ADHD issue and atomoxetine helped a lot in this area and it's effective all day long so that was a big plus for me.
I've been extremely lucky and get virtually no side effects after the initial first few weeks, which weren't fun but still better than a lot of people. I can take atomoxetine on an empty stomache but I've heard many people like OP who can't. Since I didn't get any nausea in the initial period, I chanced it on an empty stomach and that's the way I take it now.
I also had never experienced runner's high until I added extremely light runs. My regular runs would usually leave me tired so I thought that I didn't experience them like other people. I don't remember exactly why, I think it was I need to hit a higher mileage threshold, so I started trying to get any extra miles in anytime I could, including a light jog anywhere I needed to go. That's when I finally experienced it. For me, key to the runner's high was the runs that got my heart rate into zone 2-3 (approx. 60-75% max HR) but importantly weren't long runs which would tire me out. Tiredness is a big trigger for me to feel shitty so I think that's why I never had the feel goods after the runs.