Most years I train for a spring marathon and then go on to triathlon from end of May onwards. Marathon training will give you an excellent endurance base from which you can concentrate on quality running (speed-work etc). I swim once or twice a week during the winter and early spring and do a turbo or spin bike session once or twice a week with a long easy pace cycle of 2 to 2.5 hrs on Sunday, following a long run on the Saturday. Once the marathon is over I add three more bike sessions and another swim. I find that substituting a bike ride for a recovery run gives me a quicker and better result.
The above is a general aim rather than carved in stone as races (cross country etc) tend to disrupt things. I am fortunate in so far as I can sometimes do a long run midweek during daylight hours.
Your training distances will depend on what distance your proposed triathlon events are.
