Seeking the accumulated wisdom of the club on this one. I'm training for the DC tri and Nations - both Oly-distances. I'd also like to run the Marine Corps Marathon after Nations. Curious if anybody's got any thoughts on how not to overdo it and at the same time train towards a 4:30 marathon?
The cross-training that you'll be doing for tri training will provide you with a great base for marathon training. You'll want to look at your marathon training schedule and merge the long runs into your tri training (adjusting for your races and longer brick weekends). Thankfully, you're only looking at doing 2 Olys and not something longer (which, I learned the hard way last year, takes a much bigger toll on your body than you think). It's good that you know your race schedule this far in advance, because you can play with your long run schedule a little bit (to slowly ramp up), and also work in speed work early in the season. We've got a lot of veterans in the club who race tri's and road races during the season, so I'm sure someone else can chime in here.
What's your background running and triathlon-wise ? Have you been doing triathons for awhile ? Is this going to be your first or 100th marathon ? I'm not sure what a 4:30 marathon mean as it relates to you. Is a 4:30 going to be a PR or near PR time for you ? Is that a casual not too fast/not too slow time for you ?
What are your goals ? Are you trying to PR at both events or only one ? What will be your A-race ? Are you contrained by the amount of time that you can dedicate to training ? Or is the concern more with overtraining than with the amount of time that you can dedicate to training ?
Overdoing is a relative term based on your goal, experience, and background. Keep in mind that folks train for Ironmans with pretty high swim, bike, and run volume and don't kill themselves too much with the training. I think that folks can guide you a little better if you answer some of the questions that I posed.
Back in 1997, I did a sprint triathlon in Dewey in Sept. then ran the MCM and qualified for Boston. But then again, I have a running background. I would suggest that you skip MCM and do a November marathon. This gives you a few more weeks of training and recovery from Nations.
Here are a few local marathon in November to pick from:
1st weekend - Harrisburg, Pa
2nd weekend - Richmond, VA
3rd weekend - Rehoboth Beach, DC (Sat) & Philly (Sun)
4th weekend - North Central Trail - north of B-more
I've done all of those marathon 'cept Philly and even though they are smaller races than MCM, you need to think of how having more training time will help you.
I have some of the same questions as Tuan. How much experience do you have at both types of races. In 2008 I had a couple full seasons of triathlon/running under my belt and was able to do a very full season that included Columbia Oly, NYC Oly, Nations Oly and the Mountaineer Half in June all leading up to MCM. I found that the cross training for triathlon got me in really good shape for MCM and I pretty much stuck to a Half IM training plan all summer until Nations then switched into the last 7 weeks of a marathon training plan. It can be done, it just really depends what your goals are as far as how well you want to do at MCM.
Tuan brought up some excellent points that others continued - this is such an individual question. Achieving the goals that you have may be easy or impossible depending on what you've done and where you are right now with regard to training. Reaching far beyond what is realistic is an invitation to injury, frustration, and burnout. But, like Sandyc posted, you are doing well to be asking this question now and not three months from now. And if you are starting out, I would agree with vatriathlete to seek out more individual advice based on your current fitness (this person/coach/therapist could help you determine that more accurately/objectively if you are unsure) and your goals. Whether this person is a triathlon coach, physical therapist, or experienced triathlete that you can spend time with to help you figure it out - it is well worth it rather than waiting for injury or spending your time inefficiently.