There are just so many fewer women runners in the over 60 age group that it's not surprising. Remember NO ONE in that age group ran as a younger woman. (I wrote a college paper about why they should let the women run marathon in the Olympics - in 1979! This was still being debated then!)
There was a pretty steady increase in women running when Title 9 opened the doors to HS running to females and I've been at many national races were the biggest 40+ age group has been the 50-54s (who were in HS when Title 9 took effect in 1974-75). But it is really sparse starting in the late 50s and up. Some older women started up in their 20s and 30s, but the numbers were definitely smaller.
We are only now starting to get some information about how menopause and hormonal changes affect women's running. I haven't hit that yet, so I have no idea what's going to happen.
Some women just never make it back to competitive running after childbirth for many reasons. Others bounce back effortlessly (it seems) and still others bounce back fast and then succumb to injuries later. By the time you get to that age, maybe there will be more answers.
I've been able to run relatively successfully for 38 years, but I pretty much quit marathons at age 25. I did a couple attempts since then (the last when I was 43), but had 2 disasters and chickened out and ran a fast 1/2 instead for the last one. I felt a little bad about not being a "real" runner (I run xc and trails), but maybe it has helped me.
imarunr - You know I'm going to say this, but YES, cut back to ONE hard workout a week for a while (or maybe 3 every two weeks) and I am guessing you may see an improvement. Sticking to the "routine" that the younger guys do isn't beneficial for us older runners.