Extension cables?

[#1]

Hi,
I'm just waiting for my activiation and my router just arrived.
Now the nearest working phone socket is too far away for the included DSL cable and it states to avoid using 'old extension leads' in the setup guide. I've got a cheap but new extension lead linking the phoneline to where the computer is, will this badly affect my speed?
If so, will it badly affect my speed to buy a 10m DSL cable?

[#2]

A long DSL cable is much much better than a "plug-in" phone extension lead of any vintage.
(ideally get the shielded sort)
Putting the router by the master socket and using a long network cable is even better.