The walk matters! If you see a 45 minute wait somewhere, and the fastpass is valid 2 hours later, consider that you could be at the opposite side of the park during that window and would lose your time savings to the need to trek all the way back to get to the other side. You can quickly end up spending the day criss-crossing the parks for fastpasses and not gaining any time advantage.
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Yeah, basically, if you have an attraction that you really want to do that has a FP, make sure you go there first. Check out the wait time and if there's a long wait, check the FP return time and see if there's enough things to do nearby that are worth waiting for the FP. Then you've just gotta be the judge. Sometimes a FP wait would leave just enough time for a bathroom break and a snack... and bathrooms and snacks can be found all over the parks!
I'm also more likely to grab FPs when we have little ones along with. A 4yo doesn't want to wait in line for 45 minutes for a ride that lasts 5 minutes or less! If it were just dh and I, a 45 minute wait may not be as big of a deal. With kids, you've gotta remember their schedules and their tired little feet. It's more about them than us. (I can't stand people that basically drag their kids along instead of creating an experience that the kids will really enjoy along with the family.)