Perhaps this is simply an observation from my own experience, but I have known so many people that have lost weight and got in shape while living in Korea. The change in them hasn't simply been a few pounds or a short burst of training for a marathon or something, it has been a real lifestyle transformation.
Back in England, I was a gym instructor and personal trainer for a few years after leaving university and I saw many people come and go in gyms on little crusades to lose weight. The vast majority would fail miserably or experience short-term successes only to fall back to where they were before, eventually. Very few made the lifestyle changes necessary to make the real difference in their lives that they were looking for.
In Korea, however, I often see people (foreigners) making lasting changes to their health and fitness and generally in the right direction. There are perhaps a few factors involved in this:
1. More Time - talking mainly about native English teachers, most have considerably more time on their hands than normal in Korea. This means that they have less pressures or excuses from their work or social life, which means they make more time for exercise. It becomes a purpose for foreigners in Korea and something to fill the time. An English teaching job in Korea allows for much free time to ponder life and consider changes.
2. A Different Environment - when you decide to come to Korea, change is already on the mind, it is a conscious decision to take a break from the norm of everyday life in your own country. It is therefore no surprise that other revolutions occur.
3. Food - if you embrace Korean food culture you might notice that Korean food is quite a bit healthier and less calorific than Western food and they also don't tend to do desserts - this is especially true of the older generation, however the young of Korea are starting to develop different habits. The food may well be conducive to losing weight. However, even if you do not enjoy Korean food, this too can help you lose weight as some of your Western favourites might not be so available or maybe they are present, but at highly inflated prices. All this might make you eat less or differently.
4. Social Stigma - being fat carries with it more of a social stigma in Korea than in the West. What's more, you really don't need to be that fat to be called out on it (and many Koreans will keep you well-informed of your weight troubles). This might encourage foreigners to do something about it.
5. Hills and Mountains - Korea is mountainous, with over over 70% of its area being hills and mountains. This makes for some hard walking and biking - if that's how you get around - and also a nice strenuous hike is never that far from your doorstep. Koreans love hiking and the abundance of trails makes hiking so accessible. I am sure that many foreigners make hiking more of a habit in Korea than they ever did in their own countries, I know I have.
6. No Car - most native teachers and other Westerners living in Korea tend to have less need for a car than back home. Public transport is good and cities are squeezed in between mountains, so they are quite compact and easy to get around in. This all may encourage people to walk or bike to work or for other short journeys. The standard of Korean driving may also encourage people not to want to drive on the roads.
7. Outdoor Exercise Stations - whilst at the park or on a mountain you can get a more rounded workout done by utilising the equipment provided. Some seem a little strange to me, but the bars for pull-ups and the sit-up benches can be quite handy.
8. The Korean People Exercising Around You - I have quite a nice lake park near where I live in Korea and it amazes me how many people are walking or jogging around it, especially in the evening. Even when I woke up early to run at 6am, I would always find people doing their exercises in the park. I have always liked many Korean people's attitude to health and fitness. A lot of them ruin it with cigarettes and alcohol, but their thinking about improving health through food and exercise is mostly commendable. When there are others around you exercising, a bit like yawning, it can be contagious.
All of these factors work together to aid the motivation required to get in shape.
I was in fairly decent shape already before I left England, sport and exercise was already a fairly central element to my life, but even I have found myself exercising more in Korea. Of all of the above reasons for getting down to some hard-work on the exercise front, personally I think 'more time' would be my main reason. Not only the greater amount of time my job allows, but the fact that I probably have less of a social life, less TV to watch, and fewer interests generally that are more accessible to me back home in England. I find that I must actively try and fill my free time in Korea, whereas my free time just kind of dissolves away back home.
In Korea, if you want a training goal to work to, you can always enter running races, which seem to very popular and are very easy to find. If your interested, you can check out a couple of sources:
http://www.marathon.pe.kr/schedule_index.html/ (in Korean, but pretty simple)
http://www.seoulsynergy.com/seoulmarathonraces.htm (in English, but less comprehensive)
You can also set personal training goals and I tend to do this with trips to national parks around the country and hiking the full courses in a certain period of time. Jirisan from West to East in under 24 hours was a particularly exhausting one. But it need not be so far away from home; the mountainous landscape makes for some tough challenges and the trails up even the smaller mountains means there is an adventure pretty much on your doorstep.
I am privileged to have a small mountain (400m) literally right outside my apartment in Korea with a fantastically maintained trail. It means I can always do a hike for a couple of hours or a more arduous mountain run, if I feel up for it. One of the things I like about Korea also is that they often put little exercise stations somewhere near the top of these mountains when they are in a city. So I can do a run or a walk and then halfway through vary it a little more with some other exercises.
So - putting my personal trainer hat on again - come to Korea to work and enjoy the time, the food, the means, the environment and the motivation necessary to really make lasting gains in your level of fitness. It is a great place to become fit and healthy.