I agree with gents here mentioning that short hard intervals increase the risk of injury with not-so-great added value as in our young days.
After 60 (will be 65 soon) I usually do 1 interval session per week consisting of 4x4:30-4:40 on a soft trail circle in the forest. The effort by feel is somewhat around 5K race pace. But this is just because I love this exercise and used to it from my youth.