Look at the following table that has been compiled about how much contribution from the body's aerobic and anaerobic systems are used for various race distances:

From https://www.runnersworld.com/advanced/a20805305/owners-manua...

  Distance    Gastin Aerobic/Anaerobic Ratio
  Marathon    97.5/2.5
       10K    90/10
        5K    84/16
  1500m/mile  84/16
      800m    66/34
      400m    43/57

Marathon running is mostly aerobic vs running 1500m or even shorter distances like 800m or 400m.

So any loss of speed as marathoners get older won't be catastrophic versus competing in shorter races.