sorry bout the curt reply, I’m trying to get some work done… not very successfully!
Here’s the less gruff version:
there’s those folks like you who have heard ‘got to train at low intensity to foster fat burning metabolism at all times’. They train at low to very low intensities (often significantly below planned race pace), and consequently can’t increase their race pace as desired. There’s more to it, but that’s it in a nutshell.
don’t get me wrong - you train a lot of hours, so I’m sure your base fitness is good (also evidenced by your low HR although I don’t know your max HR). But in order to increase your race pace, you have to raise your training pace.
IOW: 15 mph gets 56 miles done in 3:44 hrs. 17 mph = 3:18. Combine that with the fact that you may need a little bit of time to get settled after T1, and you have problems right off the bat reaching your 5:00 goal time.
I know there’s guys like Gordo who preach a lot of the ‘go long’ stuff, but if you look at what he actually does (well, did, I guess), I believe that there’s some good amount of high-intensity work in his advice.
So my advice to you would be that even if you plan on sticking with 1/2 IM distances, to methodically pick up some higher-intensity sessions. If you check recent IM results, I believe you’ll see that recent converts from shorter distances such as Olympic Distance tend to do well.
If I had to put it into one sentence: train a lot, train consistently, and across and slightly beyond a range of situations which may apply during your goal race. Anyone who asks me for training advice gets this line. Few manage to do it for one reason or another, including myself…
Anyhow, based on your description of yourself, you’re currently not doing that, and that hurts you b/c you spend a lot of time on the bike during the 1/2 IM.
Sorry for the rambling post!
Josef