10-12 miles daily

That’s a ton of time on your feet. I’d consider switching it up. Maybe cap your runs at 1 hr and shoot for one workout per week. Might surprise yourself.

Have you considered doing ultramarathons?

An easy 10-12 miles is my favorite kind of run (when I'm in shape for it, that is). I love the phase of base training where I can just head out and put in some miles every day.

But at some point that training has to go somewhere. Fortunately, there are next logical steps for you:

Run 6 days a week

Pick out hilly routes

Select one day each week to run at a faster pace for 3-10 miles, depending on how you feel

Try to increase the distance of your run one day each week

Run to an open track and run a fast lap for fun

For most of my adult life I ran 70-80 miles per week, typically about 10 miles 6 days per week and somewhat longer once per week. Doing speed work usually just got me injured, so I didn't do it. I did quite well as an age-group runner in half-marathons off that sort of training, although I was running a little faster than you even at age 60.

I think you need longer long runs to prepare for an ultramarathon or even a full marathon, but your training might set you up very well for half-marathons.

Lots of people suggesting varying lengths of races but OP says they don't want to race.

I am not really sure what they are asking.

Honestly, yea.

If you want to get into some workout stuff, I would try some strides twice a week (4-6 x :20, 1:00 jogs) at the end of your runs. Do that for a couple weeks and then try some fartlek stuff. 6 x 1:00 on/off. Bump volume. Try 2:00 reps, 3:00 reps, 4:00 reps, etc. You don't have to go to the track to get a little faster.

But as far as goals go, idk what would make more sense than an ultra where you'd lock into ~same pace.

I used to switch up my 10-12 mile morning run by adding in some 1-2 mile faster-paced efforts, such as 4-5x 1-mile or 2-3x 2-mile. I just ran by effort, whatever felt good that day. I used a flat route for this simply to make holding a constant effort easier. I didn’t worry about exact distances or how long my recovery jog was. I just went by my watch and started the next one when I felt ready. For me, that was a lot more fun then running around a track and it got me in pretty good shape.