Consulting with an attorney will give you a good idea of the mechanics of divorce - procedure, cost, time lines, and where you will stand financially once it's over. Those are important issues.
As far as the emotions of getting through divorce, I think we all go through the grieving process. As you know, the feelings associated with grief are not linear, they crop up in different order, one at a time or several at once. That's natural. It's the end of something that you've known and leaned on for 40 years. It would be unrealistic to think it will end without some sort of mourning period.
I suppose the length of time you mourn depends on a few things. I think in a situation like mine, where you are caught totally off guard and there is no real choice to R, there is a lot of healing that has to go on post divorce. It hits us hard and hits us fast and the D seems like the start of the roller coaster ride from hell.
In your case, where R was on the table, you tried, and came to the realization on your own that you need to get out, I have to think that a lot of the healing has already occurred. The process of taking back your control and understanding your own worth has happened.
I would suspect that for people in your situation, D will likely come as more of a relief. If you describe your M as a "hell" for all these years, D may be something you will want more than some others.
Regardless of how you reach the decision, the other part of D is the feeling of freedom that comes once the grief and processing of reality is near the end. It's the freedom of being able to meet yourself again. The self that got buried and forgotten once you started to sacrifice all for the good of the marriage. You can rekindle old friendships and start new ones. Where you live becomes your own. It's amazing what paint, fabric and even a new toilet can do for your sense of independence. You have a chance to start your own traditions for holidays and special times. You have time to volunteer, take up hobbies, take a class, travel, etc. These all can be positive. and can help fulfill the need to get to know yourself all over again.
The time for a new relationship may come around one day too. However, we at SI usually try hard to convince people to hold off on that part for a while. Jumping from an M to a relationship doesn't normally work out and really robs you of the chance for getting to know you again.
Now, the one thing I did not mention is kids. If you have kids, they are older and maybe even have kids of their own. Even though you will not have to deal with visitation and co parenting and lots of the ugly shit that a lot of us in SD are forced to face, D will still no doubt impact your children. Their every day life may not change, but their family dynamic will change and you need to be prepared to hear their opinions and their desire to try to guilt you into thinking D is wrong.
There's lots more but that likely hits the broad strokes. I would spend lots of time in SD to read and ask questions. We are a good group and offer lots of love and support.