For example, today I am scheduled to do a 40 minute run in Zone 2. If you are already pretty fit, this really does not feel like all that much. What I have done for the last couple of these runs is to include a few Zone 4 bursts, just to up the intensity a little. Because the volume for this program is quite low, I think that increasing the intensity a little can only help, provided you don't overdo it and give yourself enough time to rest.
I monitor my resting heart rate and really pay attention to how I am feel, and so far there are no signs of over-training, so I think this extra little bit of intensity can only help.
So basically the, modified workout looks like this:
- Run for 20 minutes in Zone 2 -4 minutes in Zone 4 -Recover for 1 minute by Jogging slowly (do not walk) - you heart rate should drop back towards Zone 2 -4 minutes in Zone 4 -Run for 15 minutes in Zone 2
After this I usually do a bit of stretching and try to fir in a little core work and some calisthenics.
For detail on effective heart-rate training and what the different zones are, check out one of my previous articles: