Am I ready to start TRAINING?

How much of a base do you need to actually start training for a half iron man or an ironman.

I ask because I have managed to complete a half (7 hours) and a full (14 1/2 hours).  I did this mostly by trying to bike as much as a could with a little running and even less swimming.  I always trained at more or less the same pace, of course the longer I biked/ran/swim the slower my pace would get as the workout wore on. 

Now I am not a complete fool as I skimmed a number of triathlon training books as well as the begineer ironman plan on this website.  As such, I was aware that one could vary the RPE.  I really never did this, my bikes were almost always around a 3-4, my runs around a 5-6 (mostly because I found running so hard) and my swims probably around a 2-3. 

I trained this way because I felt that given my limited background in all three sports and my limited general fitness level that I would be risking injury to up my intensity.

So now I want to get better, but I don't want to hurt myself.  I am rereading all the books that I read before and getting much more out of them.  I am working on my running, adding some hills and pushing my pace significantly for my long runs, while at the same time putting in some recovery runs, which ironically are run at my old pace.  That being said, I still have the same problem on pacing my longer vs. shorter (temp?)runs (10 kish) in that I start at the same pace regardless.

I am completely ignoring my bike right now, which I know is the wrong thing to do, I just have to get over my avoidance of the trainer.  Currently I do not really have the time to do much swimming and figure my swim was, strangely, the stronger of my events.

I suppose I am looking for some advice, can I and how should I up my biking and running in order to improve?  I made a goal of a 5 1/2 hour half next August, can I make this?

Any help/advice appreciated.