59 y/o here. I've found that, for me, it comes down to experimenting with recovery days which came up on another thread.
10 years ago, I could manage a long run on Sunday, short tempo or track intervals on Wednesday, 90-120 min. run on Friday and then back to the long run on Sunday.
Now, after a few years of experimentation I've done best for about a year on 3 easy run days followed by a long or fast run on the 4th day, then a day off. That way each hard or long run comes about every 10 days.
60 track or short hard temp
Long run; for a marathon could be 3 hours with some harder running in second half of run.
Still getting slower but the philosophy is to train for a marathon because I like the lifestyle but expect to run the half unless the workouts show me that a full marathon would be interesting.
As someone on this board remarked years ago; "It's not that you can't run hard as you age, you just have to do less of it."