That'll be an "as the crow flies" estimate.
Your actual line length will be longer. However, hopefully you'd get near enough full speed, unless the actual line length is ridiculous compared to you distance from the exchange.
It is affected by more than just line length though......noise can have a big impact on the speed & quality of your line.
You could have a really good (low) attenuation value (shorter the line - lower the attenuation), yet still get crappy speeds if your noise margin is poor.
Have a read of this:
http://www.dslzoneuk.net/socket.php
I'd recommend disconnecting the ring wire (orange with white bands, no.3) from the back of the BT master's faceplate & the back of any extension sockets, as a first step in noise reduction.
It is also definitely worth getting a filtered faceplate for the BT master (if you have a fairly modern NTE5 master).
I've mentioned them before here:
http://www.cableforum.co.uk/board/34242326-post2.html
I now use an ADSLNation XTE-2005 filtered faceplate.
http://www.adslnation.com/products/xte2005.php
Also worth buying decent quality microfilters, such as these:
http://www.adslnation.com/products/xf-1e.php
Or you can get extension sockets with built-in microfilters, such as these:
http://www.adslnation.com/products/xtf.php
[If you have a filtered faceplate, then microfilters or filtered sockets are not needed for any extensions run from the filtered output of the faceplate - as the filtering is done at the faceplate. But, you may run an unfiltered extension from the faceplate, & may want to then provide a filter at the other end of that depending on what is being connected].
The quality of cable used for extensions can also affect things. Cat5e or Cat6 network cable, or decent quality "ADSL cable", is better than bog-standard "phone cable".