We have this idea that the Solar System is a vast place, and when we hear that Pluto is about 3 billion miles away or that the probe currently making its way to Pluto will arrive in 2015, our suspicions are confirmed. Even proposed manned missions to Mars talk about a 7 month transit time.
But that's more a reflection of how we choose to go. Currently, almost all(*1) of our probe propulsion is chemical... we typically accelerate in one big WOOSH (technical rocketry term) and then coast the rest of the way. You often see terms like 'Hohmann transfer orbit' - which sounds cool, but it's really just a minimum energy trajectory. Think 'slow'. Yes, the probe to Pluto has been coasting for years and will coast for many more years(*2).
There are two performance numbers of interest for a rocket, roughly sortakinda analogous to gas mileage and top speed for a car.(*3).
Here's the big 'a-ha': If we got better 'gas mileage' with our rockets, we wouldn't *have* to use minimum-energy trajectories! And the trips wouldn't take nearly so long.
In the future we'll travel using continuous-thrust propulsion. And we don't even need much thrust. If we could thrust at a continuous .03g (that is 3% of the gravitational pull you feel right now), Pluto is just over 2 months away! Talk about shrinking the size of the Solar System!
But .03g would really be quite a futuristic feat. I'd love to see a manned vehicle capable of even .0001g, which would be interesting just as a proof-of-concept; it's too wimpy to actually get us anywhere.
Things start to get fun around .001g. If we had a manned vehicle capable of .001g, we could start thinking about a 7-month trip to Europa rather than Mars. A while back I wrote a sci-fi story(*4) about the first manned mission to Ceres, and in that story, the crew ship was capable of .001g acceleration.
A century from now 'Hohmann transfer orbit' will be a quaint olde term. All manned trips will be continuous-thrust, we'll use spiral trajectories, etc. The Solar System will effectively shrink in size just like the Earth has shrunk over the last century due to telecommunications.
(*1) We have used Ion engines on one or two probe missions, everything else is chemical.
(*2) With the exception of minor course corrections. We may have (I don't know) done a burn while passing Jupiter, it is an efficient way to add speed; accelerating during a gravitational slingshot.
(*3) The analogous numbers are Isp and maximum sustained acceleration. Isp is the Specific Impulse of the rocket; measured in how long (in seconds) a pound of fuel can produce a pound of thrust. Today's chemical rockets are in the 300s - 450s range. Ion engines are over 20,000s - but today, we can't take off from Earth with Ion engines. Even if we could we probably wouldn't choose to since the exhaust is dangerous.
(*4) Since I wrote it I have come across some writings on the web that make me painfully aware I can't write worth a hoot!