Originally Posted by chasm54

If you are going as hard as you can sustain for the 34 minute segment, you should take your average HR for the last 20 minutes of that segment and use that as a good approximation of LTHR.

Then try to time-trial at your LTHR. You might go slightly over on any uphill sections, and be slightly under on the downhills, but try to stay there or thereabouts until your final push for the line, when you max out.