Stretching the limits

As you know the area that will make the biggest difference to my next Ironman is the bike.  My coach is pushing me hard on the turbo trainer (static bike) as the weather is too bad for outdoor training and the goal is to get my heart rate higher than the current level when I am hard.  This involves going into an area of huge discomfort as my lungs are screaming for air while my legs are crying in pain and my brain is telling me to slow down or I'll explode.  That more or less describes last nights training session and I did get my heart rate above the functional threshold of 157 to 161 - but it wasn't easy.
This isn't the way I will be riding in Ironman but it is about conditioning my body and mind to a higher level.  When I am riding in Ironman I will be at a lower heart rate as I will be in the saddle for 6 - 7 hours but because I have stretched my limits before the event I should perform better.
Sometimes however you have to know when not push the limits and when I was doing an impersonation of Bambi on ice this morning I had to slow down and change my route for the run otherwise I'd have injured myself.  This has also convinced me that until the temperature rises I won't be doing any more early morning runs as one accident could wipe out all the training.