Training at 6,000 ft will be slightly less effective than at 2,000, but the benefits of being at altitude the rest of the day outweigh the slight dip in fitness benefit from workouts. If you can't "live high, train low", you generally have to just accept the reduced benefit of the workouts at the slower (or less aerobic) paces. I have used supplemental oxygen, which provides some benefit but is a pain logistically, and I would be sure to include some economy reps of 15-45 seconds very fast with long, easy recovery periods to keep some speed while training at slower paces,

how are you defining "effective" and how are you measuring it?

It's not so much lack of oxygen but pressure, if anything excess oxygen can cause more damage, that is why divers do not use pure oxygen tanks

Not sure how much the terrain is the source of the difference, but in general you should not be losing that much time moving from 2000==>6000. Is going to be more like 10-15s. In either case, unless it's permanent and you are focused on open running races rather than triathlon, then I wouldn't do anything at all about it, it's fine.

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Dimond Bikes Superfan

It has been 25 years since I was living at 7,200 feet above sea level and had a running coach with a post-graduate degree in high altitude training, but since then I have lived at sea level in two different states over 5,000 asl in two different states. I learned a lot from my coach and I learned from moving back and forth from the mountains and the coast.

Yes, you get the same training effect as 2,000ft asl doing 6:20 min/mi tempo pace as you do the equivalent pace, if and only if you are doing your total workout volume by time and not by distance. So...you have to run at a faster pace for a longer distance (but same time) at 2,000 ft to get the same work at as you would at 6,000ft.

When I have trained at seal level for races over 7,000 feet I have to adjust my training plans to add more volume. If I were to follow a marathon training at 6:20 pace but set my race pace at 7:00 minute pace to maintain the same effort I would only be trained for 23.7 mile mile time. So, I added 20% volume to my long runs each week if I am training for a race at elevation.

My first marathon at altitude I was really concerned about the race. I feared that I was have shortness of breath, my lunges would get the acid burn in them, that I would fatigue early, and that I would fade at the end of the race. None of that happened. I DID use an online altitude adjustment program to adjust my target race pace and I was fine. I felt like I was running at sea level. At the finish line I had to laugh at how concerned I was about the altitude. I just said to my self "What altitude? I didn't feeling any altitude".

Tempo runs are usually paced better by feel than by a clock. If your training plane prescribed 6:20 pace and you are feeling like you are doing a tempo run at 6:45 pace you are probably fine at 6:45 pace. I always see my tempo run pace drop about 40 second/mi from the beginning of the season to the end of the season. The tempo runs usually feel a lot easer at the end of the season even though I may be doing 6:45 pace when I start a training plan and 6:10 pace when I start to peak. 7:00 min/mi seems too slow for a 4,000 adjustment from 6:20 min/mi. My training paces are a little faster than yours by I only see about a 1 second a mile difference for every 1,000 feet of elevation change. That would put me at about 6:24 if I were making a 4,000 foot adjustment. It sounds like your "hilly" course may be the bigger problem. There are grade adjusted paces too, but if you are running by feel you don't need to get overly weighed down in the adjustments. Just go and train. The fitness will come.

Note: Here are results of my last two 5K results. Elevation difference was 5,200 feet. Difference in pace was 4 sec/mi. The HR got higher on the course with more hill and more elevation. The one sec/mile for every 1000 ft of elevations held pretty close. The race at altitude was a better performance. I do my interval training by feel. My training paces were about 7-8 sec/mi slower at altitude. :-)

1) Trained: Sea Level
Raced: Sea Level
Hills: 8 feet
Time: 17:13
Pace: 5:32 min/mi
HR: 165 BPM

2) Trained: 5,200 ft
Raced: 5,200 ft
Hills: 50 feet
Time: 17:21
Pace: 5:35 min/mi
HR: 169 BPM

Aerobic metabolism will be lower at any intensity at altitude. I've measured aerobic threshold, lactate threshold, and VO2 max at altitudes between 300 ft (where my training center is) and 5,800 feet myself, but there is a plethora of research from decades supporting this. This is why we sleep high, train low, and not the reverse.

I have zero experience with divers, but there is less oxygen in the blood at altitude because the partial pressure of oxygen is lower. I have used supplemental oxygen at altitude successfully myself and with many athletes and it is not dangerous at all.

A lot will also depend on how long you will be at altitude as your body will adapt and you will be able to do the same efforts and intensity

i think the received wisdom is that the higher cost and lower output at altitude means that doing "long easy" stuff is better value than intense stuff. in other words, since you'll be working harder to run slower, you're not getting as much out of it.

from personal experience i've found that i go into the red really quickly at altitude, and once i've tipped over it's really hard to come back. whereas staying under my 'limit' (call it threshold, anaerobic, whatever) i find that i can tick along all day and get those nice fat adaptations.

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