Pregnancy is going alright... I'm tired and tired of feeling sick, but so it goes! Thanks for asking.
I believe in TCOYF it says an 11 day LP is considered fertile, but I've read elsewhere that it's 10 days. I think the reality is that since LP length can vary 1-2 days each cycle that women with an average of 10 days or even 9 days could have an 11 day or 12 day LP one cycle and conceive. I have had 12 day LP's before, but again, I've logged over 30 cycles and my LP averages 10 days and I've been successfully pregnant three times (and 2 very early m/c's that I attribute to a short LP). Another thing to consider is implantation timing. You can have early or late implantation (I believe the timing is 6-12 DPO). So if you implanted on 6 DPO then the embryo would have a good 4 days to produce hCG and convince the corpus luteum to stay intact. If the embryo implants later, say 9 DPO, that would only be one day before the corpus luteum disintegrates. And obviously if it was going to implant later than 10 days it wouldn't have a chance at all.
So there are several factors affecting how fertile a 10 day average LP truly is. Obviously a longer one would be better for TTC. And you're right, a shorter overall cycle is nicer when TTC, but not at the expense of your LP.