Even if you live near the mountains, altitude sickness is an issue that you need to pay attention to. Through a little acclimatization, you can prevent or mitigate the effects.
I used to be much more resilient. I lived at sea level, but was in the mountains enough that I never got hit. I got cocky. Because of work and family, I moved to the flatlands farther away from my mountains. So the last time I went out, I got hit with a massive headache as we were going into the second pass that day.
It took me some time to recognize what was happening. I was fine, but we had to stop that day and let me adjust. Because of that we had to cut an extra 15 mile loop that would have offered more new places I hadn’t visited.
It was still a great trip, but I was kicking myself for a while afterwards.
So what could I have done to acclimate before I went out?
For best acclimatizing to altitude go slowly.
I only spent one night at any altitude before hiking out. I did a brief hike that night and felt fine, but I was 3-4,000 feet below where I was the next day. Taking a bit more time is hard, but would have allowed us the option of adding that extra loop.
If you are not spending time at altitude regularly, you may need to spend an extra day or more before going out into the high country. Know yourself. If you are new to the mountains, take it slower. Plan to spend a few extra days hiking, running, skiing or biking. And those initial workouts need to me a bit lighter. Let your body take care of you.
If you are going to altitude higher than you can get in the lower 48 (14,000 feet or so), add more time and consider training at altitude to get your body set.
You tend to dehydrate more quickly at altitude so a little extra water will not hurt. Also, as many times altitude sickness manifests itself like a hangover, cut out the alcohol before you go out so you know what your symptoms mean. Celebrate afterwards.
Some say that taking Ibuprofen can help ward off mountain sickness by taking it a few days in advance. I have not seen the studies on this, so don’t know that.
Altitude illness can do more than slow you down. If it gets into HAPE or HACE it can kill you, so get information about it and allow your body to adapt before pushing it on your trail runs or backpacking trips.