Short Tri Season to Marathon Training

I really believe part of the great thing about this sport is to be flexible not only with your training but also flexible with your planned races.  Last season I went and I signed up for 1 tri a month starting in May and ending in Sept with a 1/2 Ironman.  Well that plan went down the toilet and I was only able to do the 1 tri in May and that was it.  I had to take a DNS in all the others.  1 was because of an ear infection 2 others I rolled my ankle training and couldn't run.  Family vacation ate up another.  By the time the 1/2 Iron came around I was no where near ready.  Fast forward to this year:

I decided I would try for the 1 tri a month again this time ending with a marathon in Oct. Still was doing the 1/2 Ironman in Sept.  Well family vacation plus 2 immediate family weddings this summer have put a kink in this years plans.  So currently I am signed up for a a tri in May and June and signed up for my 1st marathon in Oct.  In between I really cant sign up for races because of my family obligations.  Pretty much I have other plans for the weekends on all the local races including the 1/2 Ironman.

So now, how to train?  Obviously mid June after my tris are done for the year I will be switching over to a marathon training schedule and probably will cease swimming and biking.  Since I have never run a marathon before should I go for a more aggressive training plan?  I was thinking of trying Hal Higdon's Advanced 1 marathon plan.  Consists of running 6 days a week with some speed work thrown in for 1 day.  Do you think this is too aggressive for a 1st time marathoner and a recipe for injury?  I have been running 3 x a week pretty consistently since late Feb.  My long run is a little over 4 miles now as it is following a 1/2 Iron plan here on BT.

Also, for a 1st marathon should I make my goal just to finish?  I did a 1/2 marathon 2 years ago in 1:52.  I believe it was around 8:30 ave pace.  I was going to shoot for a 8:30 pace again to try and finish in under 4 hours.  I think that pace will have me around the 3:45 mark.