Half Ironman/Marathon

You are in the same boat as me.  I am looking to do a September 70.3 and have a long running event in March. 

You might look at the FIRST run less run faster Marathon Training plan for your race in May.  It is a 3 day a week running plan developed by runners that crossed over to Triathlon and noticed that they were running fast on three days of running a week with cross training than they were running 6 days a week.  LINK

That timing is suboptimal.  The marathon, especially for a newbie, is going to hurt and require some down-time for recovery (unless you walk a decent portion of it).  With that being said, if you're going to do it, I would:

1. Get swim lessons, join a masters group (if available), and work up to 2500-3000 meters per swim session 3 or more times/week.  Most of your efforts need to be in the 25-100m range to enable you to hold good form.  You can throw in some 200's if you want but keep most of the intervals short.  As you get closer to the 70.3 you'd want to dial in your pacing effort.  I would add some longer intervals to help with this...say 5x400 or 4x500 with 15-30secs rest between each. 

2. Bike...subscribe to TrainerRoad or Zwift.  TrainerRoad has workouts set up specifically for all race distances.  You'll need a bike trainer, a cadence and speed sensors.

Make sure you're properly fitted to your bike.  A bad fit will cost you speed and comfort.

3. Run...it's going to be tricky/tough to get adequate mileage in for a marathon while you're trying to build swim/bike fitness for a 70.3.  I don't think I would try a traditional marathon training plan since they generally requirement some form of speed work during the build.  I think you need to get to at least 40ish miles per week...more would be better but it would be tough with the swim/bike workouts you need to do.  As I mentioned in the other thread, the BarryP approach is a good one for spreading the mileage load over the course of a week.  Here is basically what I wrote in the other thread...

BarryP approach (in a nutshell):

He suggests building the mileage using 6 runs a week using 3-2-1 approach.  Take your weekly mileage and divide it by 10.  For a 50 mile week that would be 5 miles...that's your short run and you'd do that 3 times/week.  Double that amount (10 miles) and you'd do that twice a week.  Your long run would be 3x your short run (in this case, 15 miles) and you'd run that once per week.  All of this is easy running. 

Good luck.  You can do it if you tend to recover pretty well and you don't tend to get injured.  I'm neither, so I've only done 1 marathon and my triathlon races are generally HIM or shorter.

I have a couple of thoughts:

1.  Can you afford to hire a coach to write a plan for you?  I don't think you can use standard plans with your plan and current experience. 

2.  You need to start the marathon training NOW.   A medium preparation for first-time marathoner is 6 months.   Many plans are 4 months long, but they also assume 15-25 miles a week, so if you're not getting that mileage now you need to build up to it.   Sounds like you have some running base, which is a good, but do not wait.  

3.  I'm a little concerned about your bike experience.   You will need to almost double your bike distance and almost double the speed to meet the cutoffs.  Do you have a road or tri bike?   If not, a beginning level light-weight roadie (at minimum) would help a LOT.   You need the right equipment for this endeavor. 

4.  Your swim sounds like it's on track.   The trick with the half iron swim is to be able to execute the swim without hardly any expended effort, since it's so heavily weighted on the bike and run.   You can continue to add in swims throughout the 8 months to build on that.   Although a swim lesson/coach to get you on the right track is best.  

Basically:

• You will need to focus on the run through the marathon, using swim and bike for recovery.  Do not push the intensity of swim and bike in the last two months of marathon prep.
• Once the marathon is done, only easy running for the month following.  Depending on how it goes,  you may need a week or more off entirely before easing back.  Every one is different, but don't plan ANY intense running efforts for June.  Now is the time to spend as much time as possible on the bike.  The bike is the biggest part of the HIM race, so do not cut your training short on that.   Set aside nearly every weekend in May-July for long bike rides.    You don't need as much running for a half iron distance run, so you could even do your "long" runs (8-10 miles) on Wednesdays to spread things out.
• Lastly, you need to figure out your goals.    The best case scenario here is completion of both events.  It will be hard and you will have quite an accomplishment after.    The first marathon and the first HIM is for learning lessons about yourself.    If you have time or placement goals for either, and you'd be disappointed if you couldn't meet them, I would suggest you reconsider.

All in all, I think you can do this if you have the time and can hire a coach.   I also recommend searching the forums here for half-iron (HIM) advice, especially pacing and nutrition.  This has been hashed out many times and there's great advice around. 

Good luck!