The short answer is; happier than you and me. Retirement improves both health and life satisfaction. At least according to this American study from 2014 by researchers at Utah State and George Mason University.
They found a positive correlation between retirement and health, but it starts showing four years after retirement. It is still reasonable to believe that the retirement itself is the cause, as health often is a slowly moving state with a lot of inertia, constructed from your average lifestyle over the past few years, and not what you did last week. Retirement was also found to reduce obesity.
The effect on mental well-being was seen immediately after retirement. And what is interesting is that it lasted long-term as well. Four years into retirement, the participants in the study were still happier than before retirement. This is remarkable, as most life events that make you happier, such as winning a million dollars in the lottery, or getting promoted at work, is subject to the hedonic treadmill. This means that the happiness effect only lasts a short time, up to a few months. Then your mind adjusts to the new condition, and stops appreciating it, and you go back to the same happiness level as before the major life event. But retirement seems immune to this effect according to the observations in the study. There is however another 2014 study by Elisabeth Horner, that finds that while subjective well-being rises at retirement, it slowly goes back to your previous level over a period of five years.
So retirement seem to make us happier, but the scientists aren't fully agreeing if it lasts forever or just for a few years. But it is interesting to ask why? Why are retired people happier? My personal guess, as someone who has been at least temporarily retired (I think Tim Ferris' concept of mini-retirements is great, and I do it from time to time), is that the main happiness benefits are zero stress, and a great sense of freedom to do whatever you want with each day. This also leads to things found in the study, such as better health, because you can actually find the time to cook proper food and to exercise on a daily basis. When you work a lot, you tend to have so little time left for other things, so it is a lot harder to keep up a healthy lifestyle.
In wait for the big retirement, I would recommend that you try to save up money a few years at a time, and then take a year off and do whatever you feel like. Then work a few years again, and take another year of mini-retirement. In my experience it will make you a lot happier, and you get to tick off stuff from your bucket list; stuff that you might have been collecting for years but never had time to do because of work. During my past year's mini retirement, I've gone to New Zealand, I've learned photography, I've travelled to other places like Iceland, Italy, Madeira, the Maldives, Japan, Singapore, and I've gone on a 10 day silent Vipassana retreat. Also, I wrote a book. It was great, and I'm sure I might never have done a few of these things, in my whole life, if I hadn't taken the time off.