I am looking for an American proverb which connotes that average is better than maximum or minimum. For instance, if you work regularly you are probably in some cases more successful than people who work to their utmost or than people who are relaxed.
The closest common expression I can think of would be "slow and steady wins the race".
It does not however have the minimum-average-maximum connotation that you are looking for; it means that an effort that is sustained and not excessive will do better in the long run compared to an effort that is higher but erratic.
EDIT: another expression is "Moderation in all things", translated from the Latin "Est modus in rebus". Kudos due to @LPH from French.SE -- but I see Scott already came up with it.
Avoiding extremes is often referred to as "striking a Happy Medium". The implication is that you are finding the most appropriate midpoint between two undesirable opposing poles.
In your example, you might say: Working a 40 hour week allows me to strike a happy medium between laziness and overwork.
Of course this doesn't say that there's anything wrong with all play and no work. :-)
Pushing the envelope a little, we could mention Neither a borrower nor a lender be, which implicitly encourages a course of action between the two stated options.
Cambridge English Dictionary says that moderation in all things is "said to advise someone that it is best not to have or do too much or too little of anything".
To "keep one's head down" is an expression that addresses many implicit values in not being noticed, not standing out, and not possessing any trait that makes one exceptional or considerable. This 'blending in' is typical with the mean/average statistical body.
The 'betterness' is thus regarding 'better at avoiding the complexity of life that comes with being targeted [for anything]'.
There is one that I think fits, but because it mostly used in shortened version is not popular.
Formerly intended as a compliment, the phrase means that a person is a generalist rather than a specialist, versatile and adept at many things.
Which is commonly shorted (in the US anyway) to "A jack of all trades is a master of none". The implication of shortered version being that you must be at the maximum level of your profession (or specifically one particular part of your profession) rather than average at a broader subject.