Knee pain isn't your body "getting older" — it's the patellar tendon taking the same repeated hit without enough recovery between runs.
It's common to hear runners describe knee pain as their body "wearing out" or "getting older." The actual mechanism is more specific than that, and understanding it changes what you should be doing about it.
Every footstrike sends a wave of force up through the leg. A meaningful portion of that force has to pass through the patellar tendon on its way to being absorbed. Over a single run, that's not a problem — tendons are built to handle repeated load, and they even adapt and strengthen over time when given the chance to recover between sessions. The issue shows up when the same tendon takes that same load, at the same intensity, without enough recovery time between exposures — which describes most marathon training blocks by design. Sports medicine researchers describe this as a continuum: the tendon starts out simply "reactive" (a bit inflamed, but structurally fine), and if the overload continues without a change in how that load is managed, it can progress toward tissue disrepair and, eventually, more chronic degenerative changes.
This is why rest alone often doesn't fully solve the problem, and why the pain tends to come back once training resumes at the same intensity. Rest removes the load temporarily; it doesn't change where the load lands once you're back to full mileage. Addressing the actual mechanism means changing how the force is distributed during the load-bearing activity itself, not just taking time off from it.