If you are referring to being married for 20 years, and you are middle aged (ie you got married around 20 to 25 years of age), nope, that's not a long time. In fact, that's a fairly short amount of time.
The average long term marriage usually ends after 30 to 40 years, rare if it ever makes it past 40.
So, if that's not what your asking, you need to be a bit more specific :)
Sunflower below brought up a good point, however the connotation states that they don't exactly beleive that it happens; however, the concept of time does change when we get older.
Time moves much more differently for 20 year old's than it does for 40 year old's, at least in regards to perception. For most people of advanced age, time is of course a constant, however it can drag on without end. Defining who we are based on achievements or milestones, isn't exactly how one can properly perceive time.
What we do with our lives, and what we do with our time in regards to ourselves, dictates how we can properly perceive time.
As your question has nothing to do with marriage, and middle aged means between 35 and 45, i would surmies that 20 years is indeed a long stretch of time for such a person. However, if you really consider what your asking, 35 years - 20 is 15 years old; the perception of time between a 15 year old and a 35 year old is vast.
45 - 20 is 25 years old; the same holds true that the perception of time for a 25 y ear old person is much different than that of a 45 year old person.
25-15 is simply a 10 year difference. Because of this, we can surmies that man achieves several distinctions and refinements throughout the ages of 15-20, 25-35, and 35-45 with regards to how time is perceived.
I suppose this also has to deal with how we wish to see time. If we lead a stagnate life, where nothing changes, that time itself isn't seen much different than when the stagnation began to show itself.
However, if what we have been doing has drastically changed from age 15-35, then i would surmise that our perception also changes; one, for example, is the perception of time moving fast in highschool, and dragging in college.
The same premise holds true when you consider the ages of 35-45; if one's life has become stable, and predictable, then the perception of time from 35-45 would only changed when that life became stable, and anything thus after would be constant.
I would also assume, however, that from the ages of 46 to old age, that time is also perceived differently.
If the routine changes, it is not impossible that our perception of such a time will change.
Do i believe that 20 years is a long time for a middle aged man; that depends, is the question in relation to 20 years of monotony, or 20 years of constant change; either way it can be long time if it is taxing on the person. However, monotony is perceived as a long stretch of time; and challenge or excitement is perceived as a fast movement of time.
So this really is subjective to the experiences, etc., of the person who is asking the question.