If this is your first job and it's related to what you want to do in the future and/or permanently, I say you take it. 45 hours a week isn't that bad, although I thought anything over 40 you get paid overtime for, usually time and a half. The reason(s) I think you take it are for one, it's a pretty good starting salary, and two, you could use the experience if it's your first job and in a relevant field.
If you really decide you don't want it, I think you just politely say you're exploring options elsewhere. Or, you could just be up front with them and say the salary isn't quite what you were looking for, or perhaps not enough benefits. Who knwos, if you are qualified for it, they might give you more of what you want.
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