Get a heart monitor. Once you calculate your resting pulse you can figure out where your maximum fat burning range should be. It'll tel you when you go over vs under vs staying in that range. There was a thread on this somewhere but I think it got lost.
Here's some information I remember digging up
here that I've been following:
Heart Rate Training Zones
Heart rate training zones are calculated by taking into consideration your Maximum Heart Rate (MHR) and your Resting Heart Rate (RHR). Within each training zone subtle physiological effects take place to enhance your fitness.
The Energy Efficient or Recovery Zone - 60% to 70%
Training within this zone develops basic endurance and aerobic capacity. All easy recovery running should be completed at a maximum of 70%. Another advantage to running in this zone is that while you are happily fat burning you may lose weight and you will be allowing your muscles to re-energise with glycogen, which has been expended during those faster paced work-outs. Check out the Fat burning zone page.
The Aerobic Zone - 70% to 80%
Training in this zone will develop your cardiovascular system. The body's ability to transport oxygen to, and carbon dioxide away from, the working muscles can be developed and improved. As you become fitter and stronger from training in this zone it will be possible to run some of your long weekend runs at up to 75%, so getting the benefits of some fat burning and improved aerobic capacity.
The Anaerobic Zone - 80% to 90%
Training in this zone will develop your lactic acid system. In this zone your individual anaerobic threshold is found - sometimes referred to the point of deflection (POD). During these heart rates the amount of fat being utilised as the main source of energy is greatly reduced and glycogen stored in the muscle is predominantly used. One of the by-products of burning this glycogen, is the runners worst enemy, lactic acid. There is a point at which the body can no longer remove the lactic acid from the working muscles quickly enough. This happens at an individual heart rate for us all and is accompanied by a rapid rise in heart rate and a slowing of your running pace. This is your anaerobic threshold or POD. Through the correct training it is possible to delay the POD by being able to increase your ability to deal with the lactic acid for a longer period of time or by pushing the POD higher.
The Red Line Zone 90% to 100%
Training in this zone will only be possible for short periods of time. It effectively trains your fast twitch muscle fibres and helps to develop speed. This zone is reserved for interval running and only the very fit are able to train effectively within this zone.
Resting Heart Rate
To determine your resting heart rate (RHR) is very easy. Find somewhere nice and quite, lie down and relax. Position a watch or clock where you can clearly see it whilst lying down. After 20 minutes remain where you are, do not sit up, and determine your pulse rate (beats/min). Use this value as your RHR.
If you have a heart rate monitor then put it on before you lie down. After the 20 minutes check the recordings and identify the lowest value achieved. Use this value as your RHR.
As you get fitter your heart becomes more efficient at pumping blood around the body. As a result you will find your resting heart rate gets lower so you will need to check your RHR on a regular basis (e.g. Monthly).
Calculation of a zone value
The calculation of a zone value, X%, is performed in the following way:
* Subtract your RHR from your MHR giving us your working heart rate (WHR)
* Calculate the required X% on the WHR giving us "Z"
* Add "Z" and your RHR together to give us the final value
Example : The athlete's MHR is 180 and their RHR is 60 - determine the 70% value
* MHR - RHR = 180 - 60 = 120
* 70% of 120 = 84
* 84 + RHR = 84 + 60 = 144 bpm
My HRM has only three zones, I've set the middle zone for between 65% and 85% of maximum. That way I know the middle range will be mostly fat burning with a little bit of rocovery and anaerobic. On my mtb more than half my (2.5-3 hr) rides would be above 85% - was killing myself and not really doing anything productive. I'd pull back and found I was able to go faster, longer and found the results better.