I'm going to start training for a May gravel century at the end of March. This is my first century. I plan to follow this training schedule here: http://www.kintera.org/htmlcontent.asp?cid=54755. My concern is that I commute by bike to work, which is 6 miles each way. Tuesday through Friday's rides will be on a stationary at the gym over lunch, which means I'll be also riding two separate 6 mile rides on those days. On one hand I'll be putting an additional 12 miles in. On the other hand they won't be consecutive miles and will be broken into 3 different rides those days. Should I adjust the training miles to account for this, or just ignore my commute miles?
Take a "short cut". I commonly take a "short cut" on my 10-mile commute, riding up to a neighboring community and then down a bike trail, adding another 17 miles.
Added: The advantage to this is that you're already dressed and on your bike, so the time required will be ONLY the additional time you actually spend on your bike. Get going 90 minutes earlier in the morning or get home 90 minutes later in the PM -- no extra changing, no extra showers, no extra hauling the bike around.
I wouldn't bother adding in the commute miles. You should be focusing on getting your ride distances up, rather than nickle-and-diming a mile here or there. I went through one of these training routines for a charity ride 7 years ago; just follow their schedule and you'll get there. These things are tried and true. Hundreds of riders use them every year.
Ignore the commute miles and follow the training schedule. Maybe take it just a bit easier on your commute for the days with a training ride than you normally would.
Whatever you do, don't give up the commutes. They help maintain your fitness throughout the week. I have never been in as good of shape as when I was training and commuting simultaneously.
I did my first 100 miles in October, I didn't train enough and it was extremely painful and unpleasant. Get the miles in or suffer like hell. You have been warned.
The first time I did my century, I ride 200 km / week, leading up to the ride. After that I realised I didn't really need to ride that much, though it helped mentally. These days, if I wanted to ride, say 150 km, I just need to ride at least that long every week. Anything more than that will help finishing the ride faster. So, if you commuted 60 miles a week, I'd just throw in 40 miles on the weekends and maybe do one long ride to learn about refuelling if you never done it before.
You need saddle time for a century ride, because the only way to train your backside to be in the saddle for four to seven hours is to be in the saddle for longish rides, like three to four hours.
A rule of thumb for centuries is that you ought to be able to go 70-80% of the distance about two weeks prior to the century. I would add to that: over similar terrain. So in other words, if your century route includes 4000 feet of climbing, try to find a 70-80 mile training ride that also includes about the same amount of climbing. Try to ride that at the pace you plan do the century at, and let those training rides inform your pace for the century. In other words, if you're thinking you can do the century at 15 mph average, attempt that with your training rides, and see if it is realistic.
You don't need to regard your commuting miles as useless, however. If possible, do some intervals while you're commuting.
Another thing I might add: long rides require proper intake of nutrition. Personally, I find that I drink a fair amount on long rides. For example, I did a 60-mile ride this morning, and drank about 2.5 liters of water. To keep my electrolytes good, I always add some Nuun tabs to my water when I fill (and refill) my bottles. These tabs acidify the water slightly, which makes it far more refreshing, and they also add electrolytes without adding sugar. You can get some pretty bad cramps on longish rides when you neglect hydration, and that is another purpose of the 70-80 mile training rides: finding out how much you need to drink. You also need to make sure to keep munching on stuff during the ride, like Power Bars or some equivalent.