40 miles is enough for breaking 18 mins at 5 k.
Just do 2 hard days and another 3-4 easy or recovery runs.
You could try to run 1 threshold workout, begin with 2-3 x mile and increase gradually.
And another workout is 10 k pace, about 5-6 ks, do that 1 month, afther that try to accomplish 8 k race pace and compete with 5-6 reps by 1 k with 2 min recovery (maybe 3 by mile, but it's harder).
Every month add about 5 miles by running easy, not more and you will break 18 mins.
After every workout do some strides, maybe 4 by 200 m and 1500 m pace