Infidelity happens to the best of marriages, especially the ones who have been married for ages. Isn't it ironic, though? A person would think that after all these years, a marriage should be grounded, but something happens. The "circle" gets smaller, and someone would want to venture out of the marriage.
I would say, it's probably time to rekindle your relationship. Twenty years is a long time, and you probably just need to court each other again. Go out to dinner just the two of you, a walk in the park or do some of your favorite things that you used to do together before you had a family. I realize it is easier said than done, especially when you have so many responsibilities, but I think, it is worth a try to save your marriage.
Also, maybe, you guys just need some space, but not necessarily a divorce. Try to see how things will work out without being together for a while. Then talk again after a month or so. If you guys still feel that there is still love left, I think you should reunite. If not, and you're both set on having a divorce, then that's what you should do. But, at least, it's worth trying to keep the marriage. Even though your boys are older, it is still better to have both parents there for them; it is their foundation. Anyhow, I hope this helps. God bless you on whatever direction you make:-).