Thanks for the elaboration on paces, it seems I wasn’t careful enough reading the first time.
And yes I do think gradually extending the distance at 1500 pace could work well.
What I think would work even better would be some Igloi-type intervals in a progression.
This way you would be getting both a threshold type run effect and also the efficiency built from on pace running.
2x6x200 @ 1500 pace, 200 jog rest. 3×100 form strides with SLOW walkback rest on sets
12×200 @ 1500 pace, 200 jog rest.
8×300 @ 1500 pace, 200 jog rest.
Those would be some base-type session which could then graduate into some more anaerobic work as peak competition nears such as:
5×500 @ 1500 pace w/ 2.5 mins rest
3×800 @ 1500 pace w/ 2 mins rest
Just some ideas of mine. Not sure exactly how they would fit with decathlon training though.