One way of emulating a tougher, longer run is to do an intense workout over a relatively short period of time. This is particularly helpful if you don’t have much time on your hands as high intensity exercise can create similar fatigue levels to much longer, low level intensity exercise. I’ve often worn a heavy rucksack whilst on a cross trainer to replicate the muscle demands of climbing at high altitude, and if done properly, a high intensity, 45 minute workout can force your body into increasing your metabolism and boosting your respiratory rate.
The other great thing about a high intensity workout is the greater mental strength obtained; learning not to give up when things get tough. But remember, this only works if you really push yourself. Don’t get tempted to rack up junk miles, that is, miles which don’t have any real purpose as these will not help. Instead you must commit to stepping out of your comfort zone and pushing yourself as hard as you can.