After reading responses so far and your responses to them, I still feel like my approach would be a shorter response to him and doing it sooner than later. I would keep it short for several reasons: 1) to make sure you say nothing that you would later feel a need to backtrack from. (That is not good for either of you in this state)
2) It opens the door for him to elaborate further
3) a fairly quick response shows enthusiasm from you.
And I would just quickly write and focus on what you agree about. You can always write more later! 1) You have the same list of wants for the marriage, 2) you agree that you are looking for commitment from him, 3) Reading him write about what he feels was moving to you and was a huge step toward what you have wanted - an opportunity for both of you to get what you want in your marriage.
Oh and I'm afraid my comment about the truck will be taken out of context. I really think that getting rid of it is an important last ditch effort to show your commitment to staying married. The only reason I commented about it was because of the dynamic between Heather and her H.
I also want to state an assumption I have made about you and your marriage so my comments can be read in that context. I do not think that having your H willing to stay married to you as is enough for you. You may have gotten to this point by acting badly (kissing another man) but regardless you did need to push boundaries in your marriage for your own sake. If this is not correct, then just read my comments appropriately and adjust for their leaning.
By the way, I loved that line Corri quoted. (Dennis Quaid played that character perfectly) I am just not so sure it is most applicable to your marriage right now. There are plenty of people that believe that commitment to a marriage only means not divorcing and not sleeping with others. But I do not know that I believe that fully covers it for me, I think there has to be room for caring for the other person, respecting them, etc. And that is what I like about the analogy "Find a woman you can be in the foxhole with" - this implies that you have trust and comfort with that person - and "keep your dick in you pants when you are out of the foxhole" implies the discipline to stick with the original commitment. What I worry about for you is that your H does not show the level of commitment that you want. I guess I do not take his letter as a sign of commitment. But I still see the letter as incredibly significant and most importantly it opens better communication opportunities between the two of you than I have been aware. There was not much of a chance for the marriage while you were not communicating so this is an incredible breakthrough.
But what is happiness except the simple harmony between a man and the life he leads? ~Albert Camus