in '08 I see two marathons and an IM, plus several half-IMs and other races. All of these were done at high effort and PR or near-PR times, too. Even the elites don't race more than two marathons a year.
Ten days off isn't going to do it.. unless of course you are tigerchik's age ;-)

you need a decent break I think. Tell your friend you won't be able to race the marathon with her, but can do it as a fun run..

A good overview is here:
http://www.physsportsmed.com/...1/05_01/uusitalo.htm

There is no good physiological diagnostic for the condition - resting heart rate is an indicator, as is the variability between resting and standing HRs, but neither of these is diagnostic. The true diagnostic is a complex of symptoms which are not easily quantifiable.

Check out slowman's take on the subject too,
http://www.slowtwitch.com/...rn/overtraining.html

Joe Friel had an article on this which has disappeared, used to be at
http://www.ultrafit.com/...her/Overtraining.doc
He's funny,
"To accomplish this amazing feat you had to do too much training with too little rest, too
much anaerobic training with too little aerobic and strength base, eat inadequately, try to
train normally despite excessive psychological stress, or some combination of several of
these. Less than one-tenth of one percent of the general population is capable of attaining
such a feat. You’re an exceptional person to have done it. Congratulations!"

On the several occasions that I've achieved overtraining: falling back to hiking/swimming or biking gently/weight work/yoga, with 3-4 sessions a week, seems to maintain some fitness while still allowing for recovery.