Great info everyone.... in simplest terms I was thinking 8 weeks of dedicated marathon training (which I have never done) +2 more years in triathlon would equate to a 17 minute gain on the marathon.

Odds appear heavily against it, I may set my sights a bit lower and hopefully still finish with a big PR.

You can do it. Don't listen to detractors. They do not know you. If you are dedicated to the 8 weeks it is certainly possible.. If you were grossly overweight and completely out of shape then no. Just put your head down and try.

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Swimmer
http://www.usprotri.com/LAMASTRA.html
http://www.bodyphyxinternational.com

Maybe you can do it. I am running 4 days a week, though I had a good Spring build up to a half marathon, then a June of racing. My summer is being dedicated to a a marathon in 2 weeks. 1 long, 1 race pace / tempo, 1 track and 1 easy run. Every other day is cycling and I do at least 2 days of rehab exercises. My max week was about 68 kms I think? Usually around 50 km. I hit a 2:53 38km run 3 weeks ago as my peak. I am aiming for 3:10, though Jack Daniels has me potentially able to run a 2:54 full.

Personally when I start to hit 70+ kms a week I get injured. I know I can finish, I "should" have no problem with a 3:10, which is more than enough to BQ for me.

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Ian

It's definitely possible but 4 runs a week will make it likely. I barely qualified for Boston at 3:04 on my second marathon this spring doing 1 long, 1 tempo,10 miler, and easy 4 mile run. I was also riding thrice a week and swimming twice. I was "prone" to missing one run workout here and there. 10 mile was almost race pace (I couldn't do the race pace), long was 8-8:30/mile, and tempo sections were 6:25/mile or faster.

Average weekly mileage is 30-35 and I don't have a running background.

I decided to run and qualify 5 weeks out. I had to do a 3:15 also. Longest run was 18 and that was on a treadmill. All race pace (7:10min/mi) couldnt waste time trying to build. I am also running everyday so my shorter runs were 1mi and at most 6mi besides long run.
The marathon(mountains to beach) was downhill for the most part with 4-6mi along the beach. At the start i made the decision to run with the 3:05 group. BEST DECISION EVER. I was able to draft at times and focus on my running and making sure i was getting in my nutrition, not worring about pace.
Finished 3:07

YOU CAN DO IT!!

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http://www.TriScottsdale.org

This is why I go to slowtwitch and not runners world. Thanks everyone for the encouragement. I do think it's possible based on these latter responses and examples of others.

Swimjim, I've been in races with you before. ... you won Cleveland a few years back... I've done the Cleveland tri the past 5 years...

Based on the feedback I will add more runs at race pace and make every other week a 4 run week.

I'll throw my plan up here when ready...

I'd say it's possible, but need to at least know two things - 1). recent time for your last 5k/10k/HM, and 2) how long are your long runs now?

Reasoning:
1). You need to be just plain capable of running the speed needed to hit your time goal, and middle distance races are a pretty decent predictor. Plug in 3:15 into a time predictor (http://www.runnersworld.com/...race-times-predictor), and you get a 20:20 5K, 42:23 10K, 1:33:32 HM. Are you close or under these numbers recently? (you can subtract :15/mile from a tri run leg to get a good idea of open run pace).

2). Long runs give you two main adaptions - using more fat as fuel, and structural changes (tissues get resistance to fatigue from the pounding to run long). We've can generally get about 90 minutes of race pace running on stored carbohydrate. Then you hit the wall around 2 hours and fall apart if you haven't put in the training with long runs. Repeated long runs over 2 hours depletes the stored carbohydrate and forces the body to burn fat, getting better at it at so that you can preserve and those carbs just a bit longer. Shorter runs (under 90 min) don't cause that adaptation to the same degree. You say you've got 8 weeks, but you're last long run is generally 2 weeks before the race. So you've got 6 weeks to get from wherever you are now to a 2:30 long run, about 18 miles. With a weekly long run, you could do 14, 14, 16, 16, 18, 18 miles over the next 6 weeks. Are you close to that distance now? Rushing the long distance buildup increases the risk of injury given the weight bearing nature of running (vs. say biking).

I'm on roughly the same schedule, but one more sprint tri in 2 weeks so I do 2 runs, 2 bike, 1 swim per week. One run is intervals, the other is long. After the sprint, I'll drop the swim and one bike and add another run. That'll be 3 runs/week - a long run, a medium marathon pace run, and another easy run with a pickup at the end to maintain speed work. Shooting for 3:10, 5/10K race times predict under that and long runs at 2 hours now.

Keep in mind, we are talking about someone peaking at 30 miles per week. Meaning most weeks will be less than 30. What is a long run going to be on that mileage? 10 miles? 15? That's not going to push back much of anything for someone with the speed to be aiming for a low 3-hour marathon. And if he does a 15 mile or longer run - like 18-20+, how many other days per week is he going to be running most weeks? One? Two?

Someone who gets hurt running at over 30 miles per week is probably asking for injury by putting 1/2 to 2/3 of his/her weekly mileage into one run.

I hope the OP hits his goal. But I think he would be better served by accepting that eight weeks at <= 30 mpw is not a mileage level at which you can come up with special distribution plan to improve marathon performance. There's just not enough mileage to distribute.