do you also need to go to the gym?

I had commute problem where i was 1 hr commute by bus and didn't have time for the gym, I could have solved it by buying a car.

but instead I got a bicycle and commuted that way instead, at my most unfit I could still cycle faster that the previous commute by public transport would take, an I even enjoyed it.

Some problems are opportunities.

Once you’re accustomed to it, bicycling to work is not much exercise. It’s certainly better than nothing, but I would not see some mild cardio as a replacement for running and/or strength training.

I once heard it said about cycling that "it doesn't get easier, you only go faster." After a decade or so of mountain and road biking, I agree with that sentiment. If it's easy, that just means you're slacking. Which, on a commute I could understand -- who wants to work up a massive sweat before plopping yourself down in a chair for 8 hours?

It depends on how much effort you put into it. While one's body will adjust to the load cycling puts on it, that doesn't mean that one cannot increase the effort put in to compensate.

The commute will become shorter due to increased speed, but that doesn't stop one from putting in more effort to maintain an even higher speed.

Ok, sure, I guess some people have the luxury of a dedicated bike path with no intersections. Here in the city, I can’t get close to my (low) top speed because I hit a res light every block or two. When I used a suburban trail, it was mixed-use, so I had to go even slower to navigate safely around pedestrians and potholes.

bike paths can be counterproductive and sometimes dangerous if badly designed. they are good for learning to ride at the weekend but not worth it if you commute daily, it's better to keep your line and learn defensive riding.

it just free cardio, you can burn about 600 calories in an hour which would otherwise be sat in a car or bus.

you can use it for strength training if you do a few sprints, but the main pull of it it that you don't realise you're burning many calories unless you really try.

breathing fresh air for an hour is great too.

Then go faster. At least on the way back home you can go all out.

That's a good suggestion. I'm getting back onto long distance racing--perhaps I'll start running to the office again.