Vehicle I drive is a 2007 Honda Element that just turned 64,000 miles. I'm rigorous about keeping up on maintaining it

People constantly approach me and ask if I want to sell it because the body and interior are in great condition as are all the internal workings and suspension. Even though Honda does not require synthetic motor oil I use Valvoline full synthetic and change it every 4,000 miles with a new factory filter. My question is how long will my engine last?

It could last hundreds of thousands of miles if you keep up the regular maintenance, repair things as needed, and you drive intelligently. Most people aren't willing or capable of that.

You like to throw money away don't you… Anywhere else in the world nobody would be so daft as change oil and filter every 4,000 miles… Yes you yanks won't like it… AUDI sell the same cars as you guys buy in europe .yet they now service them every two years… Or 19,000 miles. My vans are serviced .every 18,000. And 24,000 miles .depending on size .yet our engines last just as long and we drive much faster and harder than anywhere in america…

With proper maintenance like you are doing the engine should last up to 250k miles unless you overheat or over rev it.

If you keep after the maintenance, it'll probably hang on long after you get sick of it. (Assuming you do get sick of it, which happens to rather a lot of people, though it may not happen to you.)

You never can guess at that but is sounds like you'll get many more miles and years out of your vehicle by maintaining it as you do.
Her's a piece of advice. All Honda motors have a common problem with the valves getting tight. They need to be adjusted every 60,000 miles.