This can happen: after three years of long runs, races and lots of speed work, you find out that you are getting fed up with doing a session ‘because it was on the schedule’, or because everyone else was doing it. If you already have two disappointing marathons, try considering base-building. It will open up a whole new lease of life for you. Then you will know why you are doing each run, and if you’re an inquisitive type, this will give you more pleasure and motivation
Base training doesn’t actually have to mean running. You can start your training season doing pretty much any general training (swimming, cycling, rowing, aerobics) then progress to more specific training later on. Its all about building a good cardio-respiratory system which simply requires consistent, chronic exposure to activity.
For long distance running, an increase in mileage is your best bet. Say, if you are used to running 10k for thrice a week, try doing 15k runs twice a week. Then, check your progress in time in 10k after a month. To achieve that with the minimum injury risk, keep the pace slow and comfortable.
To train for base endurance, 1hr a day of running is a good base target to work up to if you are trying to run good times. This may take some time. All increases should be very gradual and if you are not used to daily running then start with less per day than you think you can manage. And always ease back as soon as you feel and discomfort at all. If your are aiming lower then adapt it to run as many days a week as you can manage and perhaps shorter runs, say, 40min 5 days a week. But 1hr a day is best. From there you can start to add long runs and faster pace efforts (stay aerobic though).