Yes I confess I am 49 years young. And feeling like it more days than not. Only had 1 13 mile long run before the KC half marathon, I was happy to break 1:15 on that course. Plus I was only 1:25 behing Curp so I beat him in the age graded run, those extra 5 years I have on him are good for something.
Long runs were run at a hard but controlled effort. With a 48:40 10 mile PR, our 10 milers were run most times in the 53:00 to 55:00 range. hard but still being able to talk a bit. The long runs were done at 5:45 to 6:00 pace. Rarely ran the off days slower than 6:15, went more by feel. Somedays feeling OK I'd run my rest days at sub 6:00 pace.
Tempo runs often were run in a local 10k race. Run first, third and fifth miles in sub 4:50 and "recover" on 2nd, 4th and 6th miles in 5:30 pace. So I'd run a workout race in around 32:00.
Good Luck and if you can't run the quantity then make sure you run the quality. I always liked Greta Weitz's comment about LSD, "Long slow distance makes long slow distance runners".