You can modify those longer steady run of your in a way to be like a structured fartlek. We are doing in this way, 4 types for every week, about 90min:
- 1min hard(about 5k pace) 1min steady
- 7min about HM pace 1min steady
- 4min HM pace 1min steady
- progression run from short easy beginning, steady mainly, and picking up the last 1/3-1/4 part to HM pace or if feels ok to 10K pace (run to the barn type of last 500-800m)