Wellllll ... I've never personally tried it myself (=P) but AFAIK it should not hurt. Seals have a very thick layer of fat under their skin (blubber) so it probably feels like bouncing on a gym mattress at worst. Plus, their natural habitat is full of uncomfortable looking rocks anyway (example) and it would be an evolutionary disadvantage if they were in pain all the time from just moving around. Flat pavement without sharp edges probably feels less uncomfortable than those rocks.