It will help with overall conditioning, build a great base, some degree of endurance (the day after day part), and will contribute to gradual weight loss as that's 60-90 minutes/day you're active. Don't overdo additional eating, especially junk snacks and sugary drinks. I have a 10-12 mile (depending on route) each way ride with a mix of flat, easy (big ring) to moderate climbs, and descents. In addition to physical benefits, I also arrive at work wide awake and much more ready to go vs. days I drive in. I usually have a light breakfast to provide some fuel for the ride and grab something more substantial after I clean up and change.

A commute may not help as much for specific training intervals. While you're riding daily and building lots of time and miles, you're training your body to do it in 30-45 minute increments. Adjust route and schedule as days get longer to lengthen the ride, do longer rides on weekends. Like a 2 mile loop in our office park I'll sometimes ride mornings as solid finish. Ad-hoc short intervals are easier to get in vs. longer steady efforts. Traffic and lights on my route make it tough to get in more than 2-3 minutes of continuous effort without having to ease up or stop.

The "base miles" people are talking about is critical. To put a finer point on my previous post- racing and recreational riding is great but week after week, month after month, year after year, it's hard to keep it going on with a consistency. Commuting on a regular basis is an excellent discipline and one that for many of us is easier to maintain.

When I fall off my road riding schedule I comfort myself with the fact that "at least I'm getting in 100 miles a week riding to work." it sure beats the yo-yo of getting really fit and fast for a few months then going sluggish for a few months then back to riding that many people do. I can't imagine a lifetime of the yo-yoing is as good for the body as having that nice base working for you.

Any riding is good IMO.

Depends on how painful it is.

Sure. I lost 37 pounds without consuming less pizza, beer, and ice cream than I usually would and my blood pressure measures "normal for athletes and children"

Usually 2x10 zone 5a and an occasional steady 40 minutes at a threshold pace on hard days in the evenings (there's too much traffic in the morning and I couldn't just jump on my bike and work hard if there wasn't like in the evening after I'd already ridden ~50 minutes in the day) with opportunistic harder efforts after warmed up on those mornings, a lot of sweet spot rides, an easy week out of 4 and month out of 3 . I built up to that - it started at one hard day a week with more endurance pace riding, then Mondays and Thursdays, and Tuesdays/Thursdays got quicke as fitness allowed. Wednesday was a rest day usually off the bike because I didn't like riding slow enough to recover.

Then I moved closer to work and slacked off becoming inconsistent about 45 minute - 1.5 hour detours on the way in or lunch break rides.

It's a lot easier to be motivated when your choices are

1) Sleeping under your desk
2) A slow 50-60 minute ride home
3) A fast 40 minute ride home to waiting spouse and cold beer

Five hours of exercise for a cost of maybe two hours over what you likely would have been driving anyway.

It's a time machine!

Personally I like it when I change my car insurance and the agent confirms I drive 14km to work but then can't figure out why that is only 1200km a year.

I think the biggest gain is I am in just a better mood all the time at work or home.

Some helpful pointers from the USA Cycling Cat6 handbook:
1) All traffic control devices are yields.
2) You are disqualified if you put your feet down on the ground.
3) You are disqualified if it takes more than 5 seconds to hit 25 mph from a legally required stop.

Despite all the info givin try to make sure you have some variety in your route or you can get burned out on the same boring thing each day and it becomes a chore rather then something fun.

As a Type 2 diabetic, I know this won't apply to everyone, but I just did my first commute of the year this morning, and definitely noticed a difference in my blood glucose. It was approx 30 pts lower than if I drove. Not to mention that I feel great!

That.