Hi pootle. I am gearing up for my second season of triathlon, although cycling is by far my strongest leg. I'm guessing that you are aiming for Olympic distance tri if you are talking 40km bike sections? Kudos to you! I just did sprint last year, although I have half an eye for an Olympic distance later in the year, maybe.
What do you think is your weakness? Speed on the bike is down to the watts you can generate in your legs and your cardio-vascular fitness to sustain it. I am guessing that your fitness must be pretty reasonable if you are aiming for a full distance tri, so watts is the issue?
Firstly, have a look at your technique and bike set up. Is your bike set up to put you in a good aero position? Do you use aero bars? Is your seat at the right height and cleats (if you use them) in the right position?
Then have a look at your pedalling technique. Are you pedalling in a good cadence (pedal revolutions per minute) of 90 -100 on the flat? Are you simply pushing down on the pedals or could you pedal more efficiently by pulling your foot through the bottom of the stroke (imagine you are scraping something off your shoe)?
If it is simply a matter of power in your legs, then you could do some gym work to build muscle, or even do sets of squats at home.
Otherwise, just try to use your time on the bike efficiently. Work out some interval training sessions so you learn to really push yourself and work hard on the bike.
Hope this helps!