You'll get massive improvements at first. Then the rate of improvement will slow and your fitness will plateau unless you make a concerted effort to go harder. That is because the human body is efficient and doesn't like to get fitter unless taxed beyond its current capabilities.
I had (until indefinite pandemic WFH) a 31 mile round-trip that I could do up to 4 days a week, but rarely did that often because I went hard every time and needed more recovery. After commuting 2-3 times per week for half a year, I'd gone from a casual rider tired riding 5 miles if there were some slight uphill stretches to someone who could complete a solo century and hang near the front of the 18 mph pace group doing real climbs on my very first foray with group riding. A year after, I stopped seeing improvements from just riding. Nowadays, only really pushing with structured training and a powermeter will lead to a boost in fitness. Any major time off the bike and I lose that hard-earned fitness fast. I'm still far from maximizing my capabilities, and I enjoy working at it, but the easy gains everyone gets at first are long gone for me.
In contrast, I had a coworker with almost the exact same commute as me, riding with almost the same frequency. The difference was he never pushed as hard as I did, and after a year he was nowhere near as improved as I was. Though for both of us, the 1700 ft of elevation gain became far easier to handle, and we routinely passed other people when our schedules aligned and I slowed down to ride together at his regular pace, which was low Z2 for me.