Caffeine is a stimulant used by the bucketload in competitive sport because...
It prevents the action of a hormone called prolactin (released by the pituitary gland) that gives people that 'knackered' feeling when exercising.
Tests of muscle fibres via biopsy indicates that - when feeling bushed - the muscles are still packed to the gills with energy/etc., and notionally still good to go. The prolactin is the body's way of suggesting you pack up and go home before damaging something.
Just checked the first few journals that popped up on Science Direct (searching for "caffeine endurance") suggests improved performance in running time trials (around 1.5% time reduction over 5km) for trained and recreational athletes. Also, a significant reduction in experienced leg pain for cyclists... so caffeine is good.
As such, stoke up with the IAAF's maximum blood content of caffeine (last time I checked, it was 300mg/l) and enjoy the extra mileage it provides. Do bear in mind that the extra workout will need that much more recovery, too.