How far would the bike fitness take you in a marathon?

<p>Definately further than I thought.  My husband has just run 3:58 and change at Detroit.  Other than 2 long-ish runs (12.5mile trail race in Ogden UT 3 wks ago and 16miler 2 weeks ago), I can count with ONE hand for how many times he ran for the last 3-4 months.  "Training" is just not his thing - he runs when he can (which is not that often) for his health.  I know, 3:58 is not a super tough time goal to hit for a 43-yrs old man, but many train hard and long to go sub-4.  He doesn't have a running background or any endurance sports background, other than did some road cycling including a few centuries a few decades ago.  He doesn't have a 'natural' runners build either.  (In addition, his pediatrician told his mom when he was little that he had to wear leg braces to help his legs get straighter and that he wouldn't be able to run normally)  One thing he does on a regular basis is mountain biking.  He did about 10 races this year, but again with his busy schedule, he would be lucky if he could ride 2x/wk.  And he likes going hard, not a 3-4hrs of endurance ride.  He goes to yoga class 1/wk on average. </p>
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<p>Based on his marathon #1 (2007) at 4:40 & #2 (2009) at 4:20 for which he did a little more running than this year, I thought sub4 was too aggressive.  You need to 'respect the distance' right?  Of course I didn't say any of this (I need his support too <img alt="" src="http://files.kickrunners.com/smilies/wink.gif" style="width:16px;height:16px;" title="">).  The only thing I repeatedly reminded him was, to stay with 4hr pace group no matter how easy the pace felt at early miles.  I found him at 20k mark (12.4mile) and ran the rest of the course with him.  He gave me lots of credit for pacing him right but at the end of the day he ran the whole 26.2.  And he ran with 1.5mininute positive split.  To me (and I think Pftiznger's book said this too), that sounds like a perfect pacing. </p>
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<p>I would think cutting another 20min would require some more running.  But who knows?  I was just impressed and surprised cardio fitness and strength from mountainbiking would help running a long distance this much.</p>
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<p>Any thoughts? </p>
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