My estimstes of the equivalent marathon times relative to 10k are a little (not much) faster for the marathon, but those would be on equivalent footing (all-weather surface and a completely flat course). Also, as has been stated, some marathoners may never have run their best 10k because they didn't concentrate on it that much. A few times that can be compared of runners I have worked with include Jerry Lawson (28:36 10k and a couple 2:09:30+ marathons at Chicago. Ken Martin (right at 28 for 10k and 2:09:30+ marathon at NY). Peter Gilmore (right at 29 for 10K and a couple 2:12+ marathons), Larry Barthlow (right at 29 for 10k and 2:13+ marathon), and recently Jeffrey Eggleston (first marathon in 2:14:32 and over 29 for 10k). I often think breaking 2:20 is similar to breaking 30:00 for the 10k. Definitely the individual's makeup both physiologically and mentally will play a part in how the 10K and marathon relate, and probably not too many runners are focusing on a good 10k and a marathon at the same time.