Refuse to walk.

Be a bit more descriptive please --

 How long have you had her?  Is this a brand new problem or has she never wanted to "go for a walk". 

 Describe "refuses" -- she sits and won't leave the house, won't let you put the leash on, she pulls and wants to run but doesn't want to "walk".

 Is this the only time you leash her?  IS she leashed?

Have you recently changed anything?  Moved?  etc.

 Remember -- no one sees your home -- no one has any idea whatsoever of what you are trying to ask unless you describe what's going on.  Folks here are good about helping but tell them what goes on.

 Its hard for me to imagine a 7 month old retriever not wanting to walk. Good luck!

ETA: More back story would be helpful too.

 When you say "some distance" how far are you talking about walking her? What's the weather like? Does she do it in certain conditions (cold, rain, heat), but not others, or does she do it no matter what? How long has she had this problem? Does she do it if you walk in a different location or in a different direction?

IT DOES NOT MATTER WHAT CLIMATE. SHE HAS BEEN DOING IT SINCE LAST 5 MONTHS.DISTANCE SHE COVERS IS JUST 1KMS APPROX.I HAVE ALSO TRIED TAKING HER IN DIFFERENT DIRECTION. SHE WENT IN THAT DIRECTION FOR FEW DAYS THEN AGAIN SHE STOPPED.

 Whoa! Jen, was asking a simple question.

tiffy

 Whoa! Jen, was asking a simple question.

and the OP was simply answering.

Does she act frightened when she wants to turn back?  Could she have some sort of orthopedic problem that causes her pain?  Maybe a trip to your veterinarian might be helpful and mention why you're bringing her for a check up. If there's no medical reason, I would guess it's behavioral. If it were my dog, I'd probably try ending the walks earlier before she decides to turn back.  Keep walks positive and happy and shorter.

Sometimes it's not possible to determine why dogs develop certain behaviors and all we can do is work to overcome those behaviors using positive methods. 

ETA: Tiffy, not everyone realizes that the use of all capital letters is considered "yelling" when communicating on the internet.  I know I didn't and I still don't necessarily take offense if someone uses capitalization if the tone of the communication isn't emphatic.  Sometimes it's meaningless and not intended to cause anyone to take offense. :)

 My bad. Its just that the first two posts were written in normal text. Then when someone made a suggestion the reply was in all CAPS. Again, sorry.

 First, rule out the medical concerns. At her age and size, she should be able to walk that far. It doesn't sound like it is location specific, since you said it would occur even if you walked in a different direction. With those things ruled out, I'd probably go with the advice of walking slightly less than what she will do, and gradually increasing. 

I have no idea whether it was meant as shouting, but either way, the reason I ask so many questions is that it helps us to figure out what might be going on, and helps people to give the best suggestions. While we can't say for certain anything without having ever seen it, we have a better chance of getting better advice with more info.