Training for my 13th half marathon

I just put together my training plan for the Rock and Roll Half Marathon in Philly on 9/18. It will be my 8th time running that one and my 13th in total. With any luck, I will still be doing them in 10 years when I will move up to yet another age category. My goal has been to run at a pace which would qualify me for the Boston Marathon but I am realizing that unless I hire a running coach, that will probably never happen. (The other factor is that now Boston only takes marathon results, not half.) It's just a fact that, as I age,  I am slowing down. I ran a 10K on July 4th under very good conditions - flat course, low humidity, mid-70s, lots of shade. My goal was to run it in less than 50 minutes. I thought I was pushing myself more than normal but still did it in 51:40. That was faster than last year which was hot and humid but still somewhat disappointing. On the bright side, I was 6th of 46 runners in my age group so still respectable. The fastest in my age group was among the fastest runners in the whole race. If I had done 49:50, I'd have been 4th so not that much of a difference really. I'd still like to do another marathon but only if it's a destination one, like Paris, Dublin or Lisbon. If I am going to spend all of the time training, it should be worth it. All three of those are open marathons so no need to qualify. But, for right now, it's #13 in Philly.