Great to know you’re still putting in the mileage after so many years.
The answer to your question can go a few different ways.  The running speed, and how much you tax your legs can make a huge difference, especially if your giving them good recovery time.

Have you started eating leaner foods, and less pasta, dairy, or bread?

I always tell my clients, “if you want something you’ve never had, you have to do something you’ve never did.”  With that being said, it sounds like you have pushed your legs out of their comfort zone, so in return you have new results!

To increase that resistance don’t be afraid to add a steep hill in that mileage if you don’t already do so.  That incline will definitely get those tear drops of the quad muscles looking great!!
-Dominic French, Sr., BS, ACE