If you were to try to run for 25 minutes as hard as you can, but at an even pace, that would give you a general idea of your LTH.
If you are only able to go for 15 minutes and then can’t keep going, it was too fast a pace.
If after 25 minutes, you think you still have some left, then it was not fast enough.
Try to imagine the pace that you could only do for 25 minutes. That’s a good ballpark idea. But to accurately know, you would need either a lot of heart rate data, or a blood test for lactate.
But a general ballpark of that pace can tell you where you will spike. And like Carolyn said, you can do it with sprints if they are significant enough to stress you. The key is your heart rate. You can get a spike with a slower pace and longer, or a faster pace and a bit shorter. There are a lot of different ways to do it. The shortest interval I do is generally 1 minute, and that is rare. Usually I do longer intervals. But there are all kinds of ways to spike. The key is heart rate.
But, in general, I think 10 or 20 second sprints are too short for someone in good shape. Unless you do a lot of them and keep the recovery period short so that your heart rate stays elevated long enough!
Even if you are not an experienced runner, I know you are in good shape physically from all your exercise. The heart rate that causes you to spike will be different than someone else.
It takes a lot of time and practice to know it exactly! It really is different for everyone depending on the shape they are in. A workout that makes me spike would probably just be a warmup for Carolyn.