island is big enough to have 2 day hiking trip through soviet military history. and it is a national park, so follow the rules.
when you get to the island, you will be offered tickets for train. now, if it is time for train to leave in next 15 min from you arrival, go for it. waiting longer is a no-no, since you can walk faster.
for a day trip you need some planing, especially if morning boat to the island is not full and you must wait for next one. you can loose precius time.
i'd recoment at least 1 overnight stay in the island, for you to discover beaches and soviet warehouses.