"Elderly" runners seem to slow down to the same speed as other elderly villagers. Perhaps in VV2 they could slow down to a sedate jog a little faster than the normal speed of villagers? It does seem a huge difference: one minute they are zooming around the island and the next they have slowed right down to almost a crawl. Several times I have gone for Medicine 2 or 3 earlier than I intended to stave off "elderliness" as I couldn't bear to see my once speedy runners reduced to hobbling around.