An ability that turns a land into a creature also sets that creature's power and toughness. If the land was already a creature (for example, if it was the target of a spell with awaken), this will overwrite the previous effect that set its power and toughness. Effects that modify its power or toughness will continue to apply no matter when they started to take effect. The same is true for counters that change its power or toughness (such as +1/+1 counters) and effects that switch its power and toughness. For example, if Hissing Quagmire has been made a 0/0 creature with three +1/+1 counters on it, activating its last ability will turn it into a 5/5 creature that's still a land.