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I think if experienced, building it every few weeks is fine and especially if one of them is slower and then the next one is slightly quicker before going up again but agree with relative beginners orrunners who want to ensure a finish rather than a specified time, it is safer to put a shorter run in every 3 or 4 weeks. Ideally though there should be a shorter week or two in the marathon build up when you do a race or two
how much slower than race pace should my speed be on long, slow runs?
Thanks Steve, glad to know it's not just me. Brisk progressive training run it is then.
Brenda - most people prefer a minute a mile slower than their target marathon pace for long slow runs, but it can depend on the terrain and when it is in the schedule. Some runners like to do them 30-40 seconds slower and others it can be two minutes slower