If you're one of the first people to grab a fast pass usually the return time starts about an hour after the park opens. To answer your specific question, no you can't get a fast pass, line up come off and ride again with your FP right away.... well not unless somehow the line is already an hour long, which it shouldn't be (even on the busiest days - with the caveat that you really are one of the first people to get to the ride.) The only potential exceptions to this rule right now are Toy Story Midway Mania and Soarin'.
Some advice around FastPass:
1) Get your first FastPass before you ride any ride first thing in the morning. Then ride as many rides as you can while the crowds are still relatively light in the morning.
2) Every time you can get a FP, do it! The time you can get your next one is clearly marked on the ticket.
3) The return time on your FP, in my experience is unimportant so long as you return after the original time shown. Cast Members are sticklers that you do not go sooner than the return time, but you can use it hours later without issue. Often the wife and I collect several (two sometimes three depending on when the park opened) FPs while we take advantage of lighter morning hour crowds.
4) Do not be afraid to walk to take advantage of getting your next FP. People who are successful at taking advantage of the FP to it's fullest extent walk alot during a day. Several times back and forth across the parks. (It's why I only do Soarin' usually once a day, cause that's a hell of a walk from World Showcase. I like to give the FPs for Soarin away to someone while I'm enjoying a Margarita in Mexico :slurp
5) Be aware of extra magic hours, if you think you're going to be first in the parks but it's an EMH, you'll be wrong and everything will be messed up. That said, if you can be part of the EMH take advantage of them, and park hop later if you like. Parks tend to get very busy very fast on EMH days and you won't get full advantage of lighter morning crowds.
6) Be strategic about which FPs you get, for example at Hollywood Studios, EVERYONE right now goes directly to TSMM. If you don't chances are you'll still be able to get a FP in about an hours time, but it will be for late that afternoon, and if you wait 90 minutes, they'll all be gone. I imagine on busy days, they go even faster, but I haven't been to the Parks at high season in years so I don't really know. My recommendation is, go with the crowd when it comes to TSMM, be one of the first, get a FP, stand in regular line for 25-30mins and ride the ride, get off and you'll have your FP for another time later in the day. MK all depends on what you like, but the Mountains can get especially long wait times, I LOVE Splash Mountain so I usually get that one first and wait until it's hot later in the day to use it. Animal Kingdom is really a toss up between the Safari and Everest. I suggest go to Everest get your fast pass, ride it then high tail it over to the Safari and stand in regular line (it will still only be about 25 mins after opening and you won't wait for more than 30 mins to ride it.) Epcot is all about Soarin', go get your fast pass, line up right after and ride it from the regular line and go back to ride it a second time later in the day.
There are lots of places online for tips and tricks about FP. I'm sure some right here in these forums. Just do a search. You'll find everything you need to become prepared. Remember the tip to always maximize your FP (i.e. get another one as soon as you are allowed) and don't worry about the return times and you won't feel overly scheduled or rushed using them.
Good luck, and remember, the World isn't just about the rides. So take the time to just enjoy walking and experiencing the Parks. Have a great trip.
