Hi Office Boy, you might be interested in the book Faster Road Racing by Pete Pfitzinger (about £9 Amazon). It has lots useful info regarding all aspects of road racing up to half marathon distance as well as some training plans. He also does an Advanced Marathoning book. There's also a P+D (Pfitzinger & Douglas) thread on here with lots of well informed runners giving their input.
As a general rule (and having taken Mr Pfiitzinger's advice) I do my speed intervals at 5kish pace, with 50% rest - so in your case for 800m, having roughly 75 seconds rest. I also try to make my distance in the speed training 'zone' at least 4.5K So 6 x 800 = 4800m, 4 x 1200m etc.Tempo runs I do at half marathon pace and try to stay in the tempo zone for at least 25 minutes.
As someone has already said, your PB times are already pretty immpressive, so with an increase in mileage and/or more targeted training, you should make some decent gains.
PS - I cannot comment on dualthlon training, as I only ride recreationally these days (thanks to a nasty MTB leg break!). But I manage to sandwich 3 or 4 decent training runs around a bit of cycling and gym work. I believe 'easy' or recovery runs can be substituted with another form of cardio exercise.