This is how you race at alititude for 70 minutes. Don't go out too hard or you will blow up, then peg it at just under your lactate threshold. Your ceiling heart rate is much lower than at sea level. For example, I felt close to maxed out on portions of the run, but my heart rate would only be 165. My lactate threshold at sea level is probably in the 171-175 region.
USAT Winter Nationals Heart Rate Data