I had a half century round trip commute from '04-'20 (covid casualty). I started with three times a week and gradually got to where I could do five days in a row if weather/schedule/attitude allowed. Four was a nice number with Wednesdays as a rest day. I ended up with an average of about 110 commutes a year--half my work schedule. Winters were lighter (Minnesota) and involved my Cannondale hybrid with studded tires. There were some slow commutes on that. Summers were with a Cannondale touring bike. I left shoes at work and had a drawer full of snacks, but otherwise carried most everything in my pannier.

Maintaining the attitude was more challenging than the physical aspect. I spent 3.5 hours on the bike per commute on average, so there wasn't a lot of extra time and energy on the days I commuted for other activities. I kept about 100,000 miles off the car in that time frame. The touring bike saw six times the miles as the hybrid, but they went through about the same number of components. Winters here are just harder on equipment.

If you bike the full way, replace the mountain bike, or at least put slicks on it. It will take time to build up the endurance. I've read 3-4 years, and that sort of fit with my early years. (But at one annual physical, my GP said, "You have the lungs of an elite athlete!" That was about the best compliment I've received, as I definitely don't have a racer's physique nor their speed.)