Bond length and bond strength correlation
Decrease in bond length and increase in strength is only a trend, that is observed only between the same pairs of atoms, so its approximately true for C C bonds, single, double triple and similarly for N N bonds. But in general this is not true as can be seen in the following list of bonds with  approximately the same length e.g. 
$\ce{H-Br,141 pm 366} \pu{ kJ mol^{-1}}$ 
$\ce{F-F, ~142 , ~~~~158}$
$\ce{C-O, 143 ,  ~~~~360}$
$\ce{O-O, 148 ,  ~~~145}$
$\ce{N-N, 145 ,  ~~~170}$