how to transition from runner to triathlete

I'm a runner who is learning tri too, sop my answer is not based on tri experience.  Anyway, I didn't bother following any plan for biking or swimming (in part because I'm also working on recovering from some surgeries, which is touch and go, and I can't be a slave to any plan right now).  I just get out on the bike whenever I can and ride.  (I'm a long time bike commuter, but with no real experience on a road bike.)  Sometimes I ride hard, sometimes not.  Sometimes long, sometimes short.  Same with swimming.  (I started from scratch in swimming.)  Sometimes I do sets of short swims, sometimes drill, sometime 'long' (for me) swims.

My totally unsupported view is that at this very early stage in my biking and swimming careers, it is more important just to get out there and do it than to worry too much about a structured plan, especially one cobbled together from pieces of different plans.  The only training considerations that I bring to bear are generic ones, like making sure that I get some rest after a really hard workout, mixing up durations and intensities, etc. Other than that, just do it!

As a runner turned triathlete, I would recommend finding a swim coach.  My first 4 years of tri-ing my swim time for a  1K was 24:30.  After taking a break of 5 years, I got back into it.  This time, I was going to do it right and got swimming lessons from someone who coached triathletes.  My 1K time dropped to a PR of 16:44.  My 1.2 mile time was 33:45.  It was now way more enjoyable not being the last out of the water.  My coach told me swimming is all about form,  it is more like golf than it is like running or biking.  Anyways, that's my 2 cents.

Here is a bit of advice from a runner and novice-triathlete (for what its worth)

 Since you have a marathon in just two months, the running should stay the focus right now.  You can use the bike for crosstraining during the week, and just bike for the same amount of time you would have run.  You may not have much time or energy now to really start pushing the distance on the bike.  But after the marathon, you can start your triathlon training and treat the biking like the running, where you start increasing your long rides every weekend.

You may find that swimming is a better complement to the marathon training now, since it is completely different muscles.  You can easily swim on the same days you run and not compromise your running.

I am sure there are super-humans on this forum who can do long runs and bikes rides on the same weekend, but when I trained for a marathon last year, I certainly didn't want to do a hard bike ride the day after a 18 or 20 mile run.  That's just me.