Things I hear a lot.
"Doc every time I hit 10 miles I get hip pain and then knee pain, maybe I'm not meant to run."
"Seems like I do really well for a few months and then it all starts to fall apart, maybe I'm not mean to run"
"I used to be able to do marathons without any issue, now I can only do half marathons, maybe my best days are behind me."
I get a variation of the above at least once a week. What you will notice in all these comments is that it doesn't hurt to run, it hurts to run at a certain point.
What I don't think I've ever heard.
I squatted 315 last week, I jumped to 405 this week and hurt my back. I must not be meant to squat.
My plan calls for me to deadlift the most weight I've ever done this Saturday and I'm going to do it no matter what, even though I was up all night with a sick kid and I'm still very sore from my last workout.
I don't ever take days off of lifting. I'm pretty sure if I take more then three, I'd lose my strength and be way off my training schedule.
What to take away.
I see very often plans jump from 8 to 10 miles on the long weekend run. For some, this is there very first time in this training distance. This is a jump of 20%. No strength athlete would jump 20% in a PR lift. Now this is a little bit of apples to oranges, but for the most part gains are incremental. Not leaps. There is no shame in staying at a distance until your body has adapted to it.
The body needs recovery. This should be built into the program, but very often it is missing. Just because there is a "back off" run, a small dip in the training program, doesn't make it recovery. You just did less stuff. What are you actively doing to make your tissues healthier. Strength athletes do a pretty good job of doing things like foam rolling, mobility work, recovery shakes and rest days.
If a strength athlete plateaus in a lift, there is a weakness. Work on the weakness. If you can't get to 10 miles or 18 miles or whatever distance, chance are it's not your cardio respiratory system that is lacking. So doing more cardio won't solve your problem.
Don't get locked into a program. I know the long run is called for on a Saturday. Sometimes your body doesn't know it's on a precise 7 day schedule. I know your running group is meeting. I know you have to carve out special time just to do this one day. But again, your body doesn't care about any of that. In the end, it's in charge. Listen to your body.
You ran a 5k then a 10k then a 1/2. Your definitely adapting your body to keep running. But how about you run the 5k 3 minutes faster before you run the 10k. Ran the 10k. How about you run it 6 minutes faster before the 1/2. Why would this work? To run faster you must get stronger. If you get stronger, chance are you won't run into some of these issues that start to block your progress.