With the pace based plan, should I start the program with paces calculated from my last marathon time or should I derive them from a desired marathon time in order to have a plan that prepares me well for that finishing time?
From a previous marathon time or field test is the best pace to start with. Consistency and the training plan will help you improve towards your desired pace. Training at a pace you are not yet conditioned to can be a pathway to over-training, overload & injury.
Check out our resources page on setting your thresholds.