4 hr bike ride vs 1.5 hr long run

It depends. Are you a cyclist or a runner?

Both

Run in the morning, ride in the afternoon

The only runner the bike ride may benefit more is a mid/back-pack marathoner or an injured marathoner still healing up. They should still do long runs, though swapping in the bike ride some weeks isn't a bad idea. Most other runners should just run.

Obviously being a triathlete or similar changes the answer quite a bit. Ironman triathletes probably should be doing both regularly.

4 hour bike ride is a decent weekend ride, maybe equivalent to a shorter long run like 12-14 miler.

5 hour bike ride would be equivalent to a longer long run like 16-18.

The bike would be more difficult by far. In terms of better for you, if you are a runner, probably run 90 min. If I run 11-12mi in 90min, average heart of 130-133 and do the same heart rate on the bike for 4hrs, the bike be would better aerobically and use way more energy. Why would want to cycle for 4 hrs-only if I was preparing for an event, otherwise 2 hrs is plenty

Runners run. If you're asking about general fitness or you're a duathlete or triathlete maybe the ride is better. But running is what gets you fittest for running.

I do both (ran d1xc and built up to 110 mpw and a few years ago went all in on biking and finished top 30 at a 100km road race w/ close to 1000 people)

For me, if I normalize heart rate at let's say zone 2 around 72%, 4 hr ride is going to be equivalent to 2 hrs run. A lot of people tend to drift into higher zones in running, hence the tendency to expect a long run to generate more strain than long ride. But, if you are really strict about keeping the efforts (heart rate/power/etc.) in the same place, the difference isn't quite as big.

My math: 1 century ~ 1 marathon

That wasn't a race. It was a ride, or maybe a gran fondo.

Actual races don't have 1000 people, and you have to have a racing license.

It's 100% a completely different thing than a gran fondo or charity ride...

My ass hurts just thinking about sitting on a bike for four hours.

As an older runner, I think one long bike ride a week (2+ hours) provides some good no impact cross training. I would typically do it on a Sunday after doing my long run on a Saturday.

However, if you can only do one or the other, obviously you run if you want to be a faster runner.

François d'Haene did a 24h bike ride when preparing for Hard Rock 100miles, setting the record in 21h36m.

Long bike rides make sense from the metabolic and fueling point of view as part of specific long distance running training. A 24h training run would be just too taxing, no matter how slow you go and how much you refuel.

However, I do not think this translates equally to training for shorter efforts.

for me it works to not do much zone 2 in running and do it on the bike.

High cadence. And in the proper heartrate zone.

Somehow cycling give strength that helps running hills.

I am recovering from Long Covid and am increasing the running.

Then I will ride to work 18k back and forth.
At some point i will swap the bike for running.

Both are pretty intense - depends on the surface - trail or softer surfaces I would go with 1.5hr long run otherwise bike 4 hours. 1.5 hours is a long time on pavement.

this is absolutely correct. that wasn't a race and that fred definitely did not go "all in on biking". saying that is like someone saying they went all in on jogging and finished top 30 at a glitter night run with 1000 people.

Just caring about this can cost you 10-20 seconds over 5K.

Gran Fondos these days are races and taken quite seriously. These are popular and competitive because it eliminates the need to start at CAT 4 and work your way up, which is expensive and time consuming. It does not diminish the competitive aspect.

With your rational that local 5km, 10k or all comers track meet you ran isn't a race it's just a fun run.

Guess you need both if you’re competing as a triathlete, duathlete? Cyclocross the bike far weighs the short burst of portage in a race.

If this is your hobby and recreation. Whichever you enjoy more is more important.

5/10