iluvstaffies - Welcome! 
how exciting - a new soon-to-be sub-25-er for the thread 
a good session for improving would be four kilometre repetitions
i.e. measure out an accurate kilometre somewhere then run up and down it!
since your current 5k PB is about 25 mins, that's 5mins per km
so try running a kilometre in say 4:50-4:40...
then have a bit of a rest (say about 3 mins)
then run another quick km...
then rest...
then - well, you get the idea!
do one of these sessions once a week
then when you find you can do 4 in 4:40 with no probs, try either:
more reps (5 or 6)
go quicker (4:35-4:30)
less recovery (2:30 or 2mins)
some combination of the above
hope this helps
Edited: 07/11/2012 at 16:34