How old are your children ?
Do you plan to tell them of the om. You will say no, but I should imagine they will see you and him at some point and put 2 and 2 together.
Not telling them may alter their decision, they may wish to stay with their father.
If your h has not been abusive in anyway they may prefer his family unit to yours, they may not wish to have any other person to consider. And you are re -writing history, saying you never loved him, maybe he did love you more than you him, it is with this you should be careful.
You have obviously been in a marriage where you have been considered and cared for, whether you realise this or not this has increased your confidence.
You are heading for a different dynamic, where you think you care or love for someone more, you actually sound infatuated and are not thinking rationally.
This man could break your heart, if he was violently in love with you he would have stopped his life 30 years ago and fought to be with you.
It appears his circumstances have changed and you are convienient, he has clicked his fingers and you have decided to blow the family apart. This to me shows you are not in control, of this frienship and your own feelings and actions.
Even if he has been widowed, you must have idolised this man from afar, that's never a great indicator for an equal partnership.
Be also honest about this om, to yourself, yes he's the catalyst, but would you leave if he didn't exist I don't think so.
I agree with another poster who talks of women leaving to be with an adored mm, many times this leads to lonliness, regret and resentment from children. It's tough choice which I don't think you have considered, you are only considering if you should give the reasons why you are leaving.
Just consider things more carefully, don't be just an available shag or emotional crutch for this om till he finds another partner, because that could happen and your children will find you selfish for exchanging their solid base for someone you have worshiped from afar, whether that is for his emotional depth or his physical prowess.