My dad was a boxer. I was dragged down to the YMCA at the age of 12 to learn. Up until I tore a rotator cuff last year (at the age of 55), boxing was my primary fitness activity.
Our heated two-car garage is a quasi-boxing gym. A 100 lb heavy bag, a 75 lb bag, doubled-ended bag and a speed bag. A typical workout was two hours of bag work (no rests or breaks), three days a week and 30 minutes of speed and double-ended bag work twice a week. I designed and built a punch counter with a rather insensitive impact switch. This was attached to the bag chain. My goal was 3,000 counts per workout, and it took a hard shot to trip the counter. If you can do 40 rounds of fast, heavy hitting bag work without any breaks, you are extremely fit. Far more fit than your typical boxer, who would require heart massage if he tried to match you. Of course, it takes a long time to work up to that level.
My advice...
Invest in a good quality heavy bag. Avoid the cheap bags in the sporting goods store. Next, invest in a top quality set of heavy bag gloves. I suggest no less than 14 oz. Bag gloves (best with velcro closures, unless you have someone to lace you up every workout) are designed to absorb the tremendous pounding and to protect your hands. DO NOT use the light gloves that are often used on a speed bag. Injure a hand and your training is done for at least 8 weeks. Thus, you need to learn how to properly wrap you hands. This is critical. It will take you a good 15 minutes to wrap both hands correctly.
Bag Gloves, I prefer these:
http://www.ringside.com/Ringside-IMF-Tech8482-Bag-Gloves/productinfo/IMF+BG/
Hand wraps, avoid the heavy, thick velcro fastening type. I prefer Mexican type hand wraps, with a thumb loop (needed if you wrap your own hands). Apply tape liberally to secure the wraps.
I prefer these:
http://www.ringside.com/Authentic-Mexican-Handwraps-10-Pack/productinfo/AMW+10/
Heavy bags... Ringside offers a great selection. You will want to balance cost and durability. So, I suggest this one to begin with (I own this bag):
http://www.ringside.com/Powerhide-Heavybag-Filled/productinfo/PCHB/
Buy a mounting kit if you're not sure how to hang a bag.
http://www.ringside.com/Swivels/products/1247/If you have nowhere to hang the bag, you can purchase a bag stand.
http://www.ringside.com/Ringside-Prime-Heavybag-Stand/productinfo/HBSTAND/As to actual training, I suggest you locate a boxing gym near to where you live. I would avoid fitness clubs, and their girly-girl "boxing for fitness" programs. I figure I could put the typical fitness club "instructor" on a gurney in less than a minute.
Learning to box really requires an experienced trainer. Training yourself may get you fit, but you will develop bad habits and really haven't learned anything more than punching an inanimate bag.
I'm not a fan of road work. I'm a believer in training like you fight. Unless you intend run 3 miles in the ring, consider more useful way to condition your legs. Stationary bike, squats and leg curls will be enough for your goals.
Begin your home workouts slowly at first. No more than 15 minutes. Gradually build up to where you want to be. However, like any other sport, don't push too hard at the outset. That's an easy way you injure yourself.
Stretch and warm up slowly with shadow boxing. Expect to be sore a day or two after your first workout. Within a few weeks, soreness will disappear.
Finally, see your doctor for a check-up before you begin...
My regards,
Widewing