Training at 5k altitude + breathing faster?

I live/train at 7,000 ft elevation part of the year, and yes, breathing while trying to run the same pace as when I'm at 1,100 ft elevation is much more labored. The 5,000 ft you live at would have the same effect.

I don't know if you're running "too hard", but if you're out to do a LSD run and your breathing is labored or too fast to be able to speak easily, then maybe you should slow down a bit. You might want to see what your HR is and gauge your effort by that.

Like you said......  Probably a difficult question to answer.  However, I live in Denver and have some experience running in the mountains at higher altitude.  When I am training in the mountains, I do notice the lack of O2 and quickness of breath.  I have also noticed a change when traveling to lower altitudes like those in the midwest.  You might want to try training at higher and lower altitude to find out what the difference is for you from your baseline experience at 5k.  Best of luck.