The point at which the body cannot remove lactic acid as quickly as it is produced is called the lactate threshold or anaerobic threshold. It generally occurs at about 80-88% of the Heart Rate Reserve. Training in this zone helps to increase the lactate threshold, which improves performance. Training in this zone is hard: your muscles are tired, your breathing is heavy.
Pushed it into the VO2 Max zone 145 bpm at the 30:00 mark and dropped out of that zone at the 45:00 mark.
You should only train in this zone if you re very fit, and only for very short periods of time. Lactic acid develops quickly as you are operating in oxygen debt to the muscles The value of training in this zone is you can increase your fast twitch muscle fibers which increase speed.
So I spent 10 min int he Anaerobic Zone and 15 min in the VO2 Max Zone. Didn't find it all that stressful ... not easy but not hard either , somewhere in between!