I'm looking to achieve some gains in 10k performance, but realistically won't be putting in more than 40 mpw. I haven't typically had much structure in my running, but have at times done the time-to-run programs which have quality sessions of 5x2k R90, 6x1k R60, and 10x400 R60.
Current PRs are ~18:30 5k and 38:30 10k.
What's the best way to spend this somewhat limited mileage for maximum 10k gains?