personally i wouls suggest you wait and continue your base training
for another 3 months or so before you train for your half marathon..
just to make sure your body can handle it. it's possible you could
be overtraining slowly and should you train for your half now you
may get seriously injured which would really suck.
so personally i would say keep building up for another 3 months.
then start your training 3 to 5 months out from the marathon..
then keep your running X per week the same but increase the dsitance
of one of those runs... in fact to try to prevent overtraining you
may want to cut back a few of your runs durning the week as you increase
the distance on your long run.
i.e. right now you are 10k 10k 10k 10k 10k
maybe change that to
10k 7k 10k 7k <LONG RUN>
and your LONG RUN would be (based on about 3.5 months from the half marathon)
(THIS WEEK YOU TAPER.. so maybe 3 to 5K four your four weekday runs)
14 half marahton
this is just an example
i'm sure you will get other opinions as well. take them all and
make a program that you think is suitable for you