SJ - Altough the essential ingredient to being good on the bike is lots of miles, there are some short-cuts which help:
1. Pedalling technique - practising spinning around 100rpm for 30mins or so on an indoor turbo trainer. One legged intervals focusing on smooth pedalling also help.
2. Strength - building strength in your buttocks is a key for non cyclists. Get used to turning over a big gear slowly, especially up long draggy hills in your TT position.
3. BRICS - After 1 - 2 cycling sessions try to do a run around your target pace for a few miles. The main downfall for IM athletes with running back grounds seems to be knackering themselves too much on the bike and not being able to get used to running when off it. You need to get off the bike in a position you can run 'comfortably.
Other tips:
A 1hr spin on the turbo is a good leg freshener and replaces a recovery run quite nicely.
I have found 'by moving my LSR to mid week, I can do a MLR early sunday morning and follow this with the local club run 40-50m on the bike. A quick 1 - 2m run after this makes a great mornings work.
A book which might be worth digging out is 'the time crunched cyclist' by Chris Carmichael. Although aimed at road racers rather than TTers or Triathletes, it does give you some time saving training suggestions.