Or you could just review your run training to add more intensity but ensure you don't get injured.

I can't see that you'd lose muscle mass from reducing cycling. I don't believe cycling alone suddenly gives you big legs.

Most likely, you will get much, much faster even if you do 0 hrs/week biking and 6 hrs/week running (so less than 50% of previous aerobic volume). Specificity is extremely important in developing running ability. My guess is if you don't have a huge running background. Focusing on running and building specific volume will make you substantially faster if you can stay healthy.

So if you run a 22 min 5k, I'd bet you can get down to 19:30 with that small change. Or if you run a 19 min 5k already, a 17. It's that big of a difference in my experience.

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David Roche
"The Happy Runner" book: https://www.amazon.com/...Longer/dp/1492567647
Coaching: https://swaprunning.com/

Bottom line....Yes.

Runners don't train a huge number of hours (due to the impact) compared to swimmers and cyclists.

Running D1 in college we didn't spend over 90min. running per day. I'm oversimplifying it, but at 6:00/mile, running 70mi/week would only take 7hrs.

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ECMGN Therapy Silicon Valley:
Depression, Neurocognitive problems, Dementias (Testing and Evaluation), Trauma and PTSD, Traumatic Brain Injury (TBI)

I don't think the OP will get that big a boost unless he's a pretty new runner and has never run seriously for more than 4-5 hrs/wk in the past.

For the short stuff (5k-10k), the bike training does crossover some to running, and if you're lucky enough to be born with good run legs, the cardio gained from the bike can even express itself on the run.

For me, I used to be a pure marathon runner, and have been a triathlete for at least 6 years since. If I do pure Pfitzinger style run training at 70mpw, I'll run 5ks in low 18s. As a triathlete, I've found I can 'fake' my way to not too far off by biking 5hrs/wk and running only 20mpw and run a high 18 standalone 5k. It's a minute or less for me for 5ks (definitely closer to less), assuming I'm training hard on the bike in comparable more more volume than I would running.

^ THIS!

I think the difference is in the assumption I'm making, sorry! I'm guessing the OP never focused on running like you did. If that assumption is true, there are those massive gains to be made.

Disclaimer: as always, like Jon Snow, I know nothing.

(that should probably be my signature line)

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David Roche
"The Happy Runner" book: https://www.amazon.com/...Longer/dp/1492567647
Coaching: https://swaprunning.com/

As it's already been stated, this really depends on your running background. I offset some recovery runs during marathon training with bike rides. I still run 4-5 days a week and ride 2-3. I don't do any hard rides during running season. I'll peak at 40-50mpw for a marathon. I've ran as much as 80mpw with 0 riding prepping for marathon's in the past and am faster now than I was then.

No matter what distance you're trying to get faster at, you're going to need to run more than 4mpw.

Cycling turns my legs in to toothpicks. I think that sort of thing depends a lot on your morphology. A good friend of mine, who started cycling at the same time as me, had his legs just explode in size from cycling, while mine shrank. He is naturally more built, with very muscular legs, while I am tall and lean. Funny thing is, he started cycling in the hopes of slimming his legs down, while I secretly wanted bigger legs. Back to the drawing board I suppose!

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Long Chile was a silly place.

likely you dont train hard enough and eat in caloric surplus to grow the legs

Triathletes put far too much stock in "aerobic time" and cross training. This should not even be a question. If you want to be better runner, you don't hop on a bike, you run more. Barring injury, decreasing cycling and increasing running by a significant amount is obviously going to improve your running. You can quibble over the degree but yes it matters. You don't have to be new to the sport for that.

I'm also tall and lean, but find that cycling while simultaneously run training bulks my legs up a little bit. Not much, but enough to be noticeable. I imagine if I cycled only they would gain considerably in size, but have never tried it. Also, for lack of a better term, cycling "hardens" my quads. When I don't cycle for a couple months and only run, they don't feel as toned. I am not sure why this is, I imagine it is from the added recruitment of cycling-specific muscle fibers.

Thanks all...that's encouraging. Looks like I have a new project. :)

this is interesting - for me i think it's the opposite. the short, fast stuff requires mechanics and sharpness that i only get from serious dedicated run training, whereas the longer (and slower) the run gets, the less it requires that stuff and the more i'm able to count on general fitness (including from stuff like cycling).

-mike

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http://howtobeswiss.blogspot.ch/