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I guess it all depends on your aims and what intensities you do them at
take Ironman triathletes as an example - swim, then 180km bike followed by a marathon. for the top male pros they will (course dependant) knock that bike leg off in under 5 hrs and will then knock out a sub-3 marathon. but they are trained for those intensities and you'll probably find that their stand alone marathons are probably around 2:15 - 2:30 so not hugely faster
my best for those is a 5:30 bike followed by a 4:54 marathon - my marathon PB is sub-4. so quite a big run difference time. so for your average person, cycling does affect run speeds if the 2 are combined.
but otherwise, yes, cycling will help your running overall as it's another form of exercise with benefits as it's non-load bearing so less detrimental to most. but it's getting the balance right that's key and it takes time to get to know what works
I've always found it a lot easier to run after cycling long distance to the other way around, I think running puts a lot more strain on you muscles!