I have heard varying opinions on the subject of "How often to run 42.195km at close to max MP"
1. 1-2x/year only. The damage from running that far is considerable. These are all-out races, at 100% of MP.
2. 2-3 "walk through" marathons @ ~95% of MP, in addition to the 1-2 maxed out efforts. The rationale is that adaptation can occur, with sufficient rest, and so these "submaximal" efforts are quite benefical to overall training goals.
Most elites seem to do the *equivalent* of a marathon in workouts a few times a year (e.g. Ryan Halls 22-23 miles w/ the last 10 in 49). However, many a runner has crumbled under the rationale of, "well if Ryan Hall does it, then so should I"