I don't think you have to resign yourself to getting slower in your late 40's/early 50's.
I just went through a major 2 year slowdown (I'm 48 now). In the past 2 years I went from the high 16's in the 5k to mid 18's. My high 16 two years ago came during a marathon buildup a couple of years ago when I averaged 70 a week for 3 months, after running 40-50 consistently for about 6 years.
Since then, my mileage has dropped back down to 40 or so, and I feel like shit on almost all my runs. My races feel like shit, and my legs feels stiff and old and slow. I used to be a wickedly fast sprinter in high school - literally felt like I was floating all the time. Lately, these 18 minute 5K's feel like I'm breaking apart pavement.
However, about 2 months ago I decided to jack my miles back up to 70 (just did 80 this week) and for the first time in a LOOOOONG time my legs feels great and I'm running 6 minutes miles relatively easy. The first 2-3 weeks of 70 sucked big time, but out of the blue suddenly every run felt great. In fact, last night I did 12 to finish up an 80 mile week, and my last 6 were on the track - all low 6's. 2 months ago I couldn't race a 5k that fast.
I'm not kidding myself that I'll somehow run the low 15's that I did in high school, but there's no doubt I can still run as fast as I did before this last 2 year slide. I think it just takes a major training change. I try to add in strides whenever my legs feel light and loose, which unfortunately is pretty rare. I like doing them on the down hills to minimize the stress on them (gradual downhills I mean).
So, if you have the time (and most of us don't) and aren't ready to give in to getting slower, go for a major bump in miles. Give yourself 3 months. Don't worry about pace - just let it come. My first 2-3 weeks were so ugly it wasn't funny - ran a lot of 9 minute miles. Since then, it's rare I'm over 8. Mostly 7:30's. Two months ago that felt excruciatingly fast, now it's an easy run. Start all runs slow and ease into them. Run twice a day if you can. Just give yourself a couple of months to adjust, and see if it helps.