|15k X 3 = marathon|
multiple 15k time by 3 = marathon
Using my Boilermaker time (hot, hilly) for each of the past 3 years, multiply by 3 time as been within 1 minute all 3 years. Hold fairly true for elites too (42' X 3 = 2:06)
This is what I was going to say, but a little less detailed.
I was going to say that since a marathon is about 42.25 k's. Then accounting for fatigue, you could come pretty close to a predicted time of a complete marathon from a 15k by multiplying by 3. You must consider you would go through that first 15k in a marathon , and so forth.