Again, not being a physicist, I have to depend on other's words and thoughts. This is a short paragraph from the wikipedia entry on time dilation:
In theory, and to make a clearer example, time dilation could affect planned meetings for astronauts with advanced technologies and greater travel speeds. The astronauts would have to set their clocks to count exactly 80 years, whereas mission control – back on Earth – might need to count 81 years. The astronauts would return to Earth, after their mission, having aged one year less than the people staying on Earth. What is more, the local experience of time passing never actually changes for anyone. In other words, the astronauts on the ship as well as the mission control crew on Earth each feel normal, despite the effects of time dilation (i.e. to the traveling party, those stationary are living "faster"; whilst to those stood still, their counterparts in motion live "slower" at any given moment).
I interpret that to mean that both the traveller and the "stationary" observer experience time the exact same way as they always have. If you have a trip that takes 40 years then that is what you will experience. However, for the "stationary" observer your trip will appear to have taken some multiple of 40 years and therefore you will be younger and they will be older when next you meet. I don't remember reading anywhere that if a traveller is able to approach something close to lightspeed that they will somehow magically reduce the amount of time needed for the trip.
If you are a physicist and you know of things that I don't then I will just have to take it on faith that you are correct but I honestly cannot remember ever reading about such a thing as you describe. I do remember reading that another way to achieve time dilation (which is a pretty nifty way to travel forward in time at an accelerated rate relative to an observer) is by getting unrealistically close to a super massive object such as a neutron star or black hole. The problem with that plan, though, as I understand it, is that you would have to somehow overcome gravitational tidal effects which would be wanting to tear your body (not to mention your vehicle) apart as you orbited that mass.
Getting back to the original problem: Your destination is 40 light years away. Let's say that you can travel there with an average speed (includes acceleration and deceleration) of 1/2 the speed of light. Your total travel time is 80 years to get there and you will have aged 80 years. Period. If you can travel at an average speed of 90% of lightspeed then you will arrive their having aged 44.44 or so years and so on. In each case you are going to arrive at your destination having experienced something greater than the 40 years that it takes light to get there since you cannot exceed or even match that velocity. But that is the time that you experience. The stationary observer, on the other hand, who is watching you believes that you are taking a very, very long time to get there. You, looking at the stationary observer, think that time is moving very fast back on Earth. In reality, both of your are experiencing the effects of time the same, both aging at the same rate as you always have but just not in relation to each other.
Bottom line is, though, that you can't exceed the speed of light. So, if you need to go somewhere that takes light 40 years to reach then you will get there in some amount of time that (for you) is in excess of 40 years and for a stationary observer is going to take some multiple of that depending on your speed.
And, yeah, who knows what happens technologically back on Earth while you are moving at some percentage of C? Someone could somehow figure out how to develop a FTL drive and beat you to your destination.
And, yeah, for the pioneers who discovered the New World or for the settlers who pushed West, they did more or less leave everyone behind for a very long time but since velocities never got much over the speed of a galloping horse they could always return home and find everyone pretty much at their same relative ages. When we talk of interstellar travel, though, it's kind of a forever proposition short of FTL drives and communication. Sort of the Rip Van Winkle effect for the travelers.
