Recovery. A concept so many of us struggle to grasp, and yet without sufficient downtime you will never improve as a rider.
An essential component of the principles of conditioning, it may seem counterintuitive but it is during the downtime that you allow your body to adapt to the training load and you get the real benefits of the training.
An excellent article from Huw Williams explains here via the overcompensation model.
http://roadcyclinguk.com/riding/training-for-results-train-hard-recover-stronger.html#SpPRdcoxZVeR3fwe.97
How many of us train 'quite hard' all the time, plateau or hit burn out? Prioritising recovery means you can hit the intense sessions hard, and use the gentler efforts to be sociable, explore and remember why you love riding in the first place. Note, if you spend the rest of your life in a whirlwind you'll want to make time to nap, actually stop still. Trust me, people will be glad for the improved mood!
From now on I'll be using my heart rate monitor alongside a healthy dose of realism when planning intense sessions around work and social commitments. Adapting my training around my real calendar is smarter, and far more achievable.