Born to run–
I too have a bad correlation on my 5k to marathon time. it’s really about two different trainings. Infact if your training properly it should be hard to PR in a 5k when training for a Thon and vice-versa . I’m just starting to learn this. You body is not use to running slow you need to teach it from the ground up . What I mean is you have been running anaerobic a lot more than you think. Your training your body to use Glycogen for energy and it probably feels good and easy when you do this for awhile. When running in a thon you need to teach your body to burn fat , so that you will store your glycogen for the last 10k —when you really need it. Your even running out sooner at 15 miles. I know this is hard . To teach your body to run slow is just as hard to teach it to run fast. It’s just different. My correlation is 4:57 mile 17:10 5k 28:58 8k – 1:21 1/2 thon , then ouch 2:59:30. I was running my slow runs at 7 :45 -8:00 for the most part. I was also training above LT pace all the time. So guess what I actually felt better running 7 min miles for 10 miles than 8:00 min miles. So I had to re-train my body to use fat as an energy source. So I went on a mission this winter to learn how to burn fat not Glycogen by running slower and longer. It was very hard at first running 9 + min miles for 90-120 + mins. But over time I got use to it. Now I have a big base and am clearing lactate a lot easier when my body is building it up. The cool thing is you don’t really lose much speed. Any speed you do lose can be worked on in a few weeks and you have a great base to work with . Strentgh = speed over time. Knowing what I know now and if I could pass something on it would be to learn to run long and slow, Build strentgh and then add speed. I know it’s hard to understand and it might not make sense right now but teach your body to run slow for the thon and it will pay big dividends later down the road.
Good Luck 😀