You get the same benefit from a long slow run as you do from a long fast one and you’ll recover faster from the slow run. If you go further than 20 miles in the long run, you will improve your marathon stamina dramatically but you must run slowly. Remember that running more slowly will help you to recover faster and keep the legs ready to do speed sessions on the following weekend.
Liberal walk breaks will also speed recovery from long runs. Those walks must be taken early and often to give your legs the relief needed. Also, make sure that the maintenance runs during the week are done slowly enough. Sometimes a slight bit of fatigue will appear on the second or third day after a long run or a speed session. Take it very easy if that happens.