[reply]Running fitness translates better to both the bike and the swim in that its the best to reduce your lactate threshold, its a better HR workout, it burns fat faster, and it requires a lot less time to do...shoes on out the door.
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this is true as far as it goes, but leaves out the injury problem. I'd prefer to run every day, but I can't tolerate it any more. So I do a 15 mile TT on the bike with a warmup and cooldown, takes less than an hour, and measurably improves my bike speed.

Also, I'm not convinced that running can make you a better biker. I have found that my swimming speed tracks very closely with running speed, even when I'm not swim training - that is, if I'm running well, I'll be swimming well too, regardless of how much swim training I'm doing (though with a basic minimum of at least 1swim/2 weeks). I have not noticed the same effect with biking, though. I need those TTs to get faster on the bike, even if my running fitness is good.