How far/how hard?

[Peter Armstrong]

15k running, its more like riding 50 miles. Times are not comparable running to riding.

 

[JasonHolder]

Running fitness does not copy over to cycling and there is no benefit to having a good running background.

 

[Shut Up Legs]

I think general fitness is always an advantage when switching your form of exercise, so I think there is a benefit to having a good running background. As others mentioned, saddle-soreness will most likely be an issue to start with.

 

[w00hoo_kent]

It might not give you cycling muscles, but it's going to help general fitness which has to help somewhere. Someone who exercises is going to have a head start over someone who doesn't and running is a sport that rewards light people just like cycling, so getting fit for one is going to have some knock on.

 

[50000tears]

Do not agree for a second that running fitness does not translate to bike fitness. True that different muscles are employed, or at least he same muscles in a different way, but fitness is fitness. Both develop aerobic fitness and that is what riding and running are all about. For my own example my friend at work is a very strong local runner and cycles once every 2-3 months if that but when he does he wipes the floor with me and could easily keep pace in my club's fast group.

 

[bpsmith]

Just the first link of many on Google:

http://running.competitor.com/2014/06/training/study-run-faster-to-ride-better_43375