Don't worry, you've got time and with the base you've mentioned, you can do this. In addition to your commuting, you should be doing one long ride on the weekend. Start with a distance that is long but "doable". I would suggest starting with 50 miles, unless that isn't far for you or seems like too much. keep adding 5-10 miles to your long ride each week until you're up to about 75% of your goal distance. Plan rest stops about every 20 miles, as you adjust to the distances try and go further between stops and keep thme short. Get a couple rides in at this distance before your event. Start cutting back the week before your event to rest up. I would also take advantage of your commuting days. I would use them to recover after your long day and push the tempo when you're feeling fresh. You may want to choose one day a week to take a long route on your commute. During this training time it's really important to train yourself to eat and drink enough on the bike. If you don't already have energy food and drink of choice, start experimenting now to see what works for you and let your body ajust. I wouldn't worry about the weight too much, you're not going to lose 20 lbs in the next 11 weeks, and I've seen plenty of 200-250lb guys on centuries and doubles.