LSD (long steady distance) sessions are the core sessions for endurance events and can’t be missed. Some people colour code their training schedules in a traffic light fashion. Those in red (LSD sessions) cannot be dropped, amber sessions shouldn’t be dropped but the odd one isn’t too detrimental and green are the sessions that can be dropped if you need some flexibility around your week. Don’t think that green sessions are there to be dropped, as all training programmes have been structured for a specific purpose and to allow the person using the plan the maximum chance of achieving the intended goal, but it is also recognised that flexibility is necessary and this colour coded system does allow you to quickly determine where you can have some degree of flexibility. However, whilst the traffic light system is a good quick reference guide you should understand the basic principles of the plan and what the various sessions are trying to achieve. This way any session dropped has been done in an informed manner rather than ad-hoc and you should also be able to work out how to re-work that training week’s sessions in order to minimise the disturbance of the missed session/s.