The question of pacing is something that I have to deal with across all endurance sports, for short and long events. We will look at this from a racing view point mainly but will touch on people wishing to complete rather than compete.
Let's start simple and say this is not your first season... You can look back at your previous season's data, be it power or heart rate. Find your best race and look at your average and there you go! This data is key to helping with your intervals as by working above your race average, you can overreach and make your body adapt therefore enabling you to increase your average pace. This approach can be used in long distance as well as XC, we don't expect as higher power or heart rate though on the longer distance.
Next we can look at Sport Science testing or FTP, Functional Threshold Power, what we gain here is our lactate threshold and turn point. Our threshold is when we start to accumulate lactate in the muscle and turn point is when our body will no longer remove the lactate and so over time will cause us to stop being able to pedal. You do not get as accurate readings by doing a 20/30/60 min FTP with an equation at the end, it's good but not as good.
Once you have these figures, in theory you can set off and race at given efforts and not blow up (so long as nutrition and hydration are taken care of.)
Our next method is experimenting in training... This works well for shorter races but it's not so easy for 24hr or stage events. This may involve going and time trailing your local route of race length and seeing what you can hold (or known as bench mark). You can nearly always allow for 5-10% performance increase from training session to racing due to positive anxiety. If you are using heart rate, you can allow 5-10 beats above when racing as heart rate is affected hugely by anxiety (positive and negative).
I had a great conversation with one of my athletes, Steve Day who is the World 24hr MTB champion (single speed) about pacing as some guys were asking, basically you learn it through your training and racing, looking at data and learning how you feel, so when you're training, listen to your body and learn. At the end of a ride look at that data you have collected from the expensive bike computer and see what efforts you were holding on climbs, flats and downhills.
A really important note for long and short distance racing is knowing how much you can push before you go 'pop'. Everyone will have a different level of intensity that they can hold and how many times they can ramp it up. For example an XC race with 3 climbs in each lap that you will ride 5 laps, if you work too hard on the first 3 laps you may pay the price in the race by 'FLAT LINING' - this term means that our mind wants us to keep going but we cannot elevate our heart rate anymore and so no more blood/energy going to our muscles. This point is key for people who focus on power all the time, your heart is your engine and you will not produce the power if it will not increase in beats. So gage your efforts and watch how your heart rate may drop towards the end of a race, this means a loss in performance.
For 24hr and Stage racing, all the above applies but if you over cook it the negative effects will be worse. You tend to ride much closer to threshold on longer efforts and on the climbs you may hold back more so to save the energy for the end. 24hr racing is won on a solid consistent pace, having World, European and National champs under my guidance means I have studied lots of data on this and being able to hold that effort consistently is key and trough specific structured training.
For people who are looking to complete an event, this is guided by your sense of comfort... If it feels too hard it probably is and so hold back, if at the start it feels ok chances are it may still be too hard so back it off just a bit and get to the end, you can then start looking at what we have talked about previously in race data.
We are always learning as athletes and coaches, no one has all the answers (if they say they do trust me they don't) I love this job as I am always finding new methods and ways to get more from my athletes, so analyse what it is you are doing and learn.
For more info on pacing please contact firstname.lastname@example.org