Leaving a husband whose only vice is that he is boring seems a poor reason to renege on the mutual commitment and loyalty that led you to battle through life together for nearly 20 years.
Dullness is entirely subjective. What is interpreted as dullness in a man by one woman will be described by another as quiet charm. Is your husband dull, or are you just bored by him? Would you find your older lover equally boring if you had been with him every day for 20 years? By then, you would know all his jokes and stories by heart. Furthermore, within ten years, your older man will be a septuagenarian with a 50 per cent chance of being impotent, and no longer able to be the skilled, considerate lover. His physical and mental powers will be beyond their peak and the first signs of intellectual inflexibility, emotional blunting and forgetfulness may well be showing.
By then, you may remember your husband with some nostalgia.
So what's the point of this post? In my humble and lay opinion, sometimes it takes less energy to "fix" a boring marriage than it does to renege on one's promise to love and cherish one another in order to embark on a new relationship. If the pain, chaos and upheaval that result from a broken marriage are worth the effort, then leave. But if you are merely bored ... look inside. Nine times out of ten, that's where the true problem lies. Horses seeking excitement bring their problems with them to that greener pasture.