Think of a piece of playdough; if you roll it out to be twice as long, it necessarily becomes much thinner, making it harder for 'current' (or energy flow) to pass through the constricted path.

n-squared rule: Stretch by $n$, resistance grows by $n^2$.

The resistance $R$ of a conductor is given by $R = \rho \frac{L}{A}$. Since the volume $V = A \times L$ remains constant during stretching, if $L$ becomes $nL$, then the new area $A'$ must become $\frac{A}{n}$. Substituting these into the formula, the new resistance $R' = \rho \frac{nL}{A/n} = n^2 \rho \frac{L}{A} = n^2 R$. Here, $n=2$, so $R' = 2^2 R = 4R$.

Always remember the difference between 'adding an identical wire' (parallel/series) and 'stretching/reforming' the existing material.