Please keep in mind that all advice here is relative(hint above).

If you want to ride a century in March then I would continue your commute and ride for fun this summer. Take some time off of the bike end of September early October to freshen up and focus on the goals ahead. In October start to focus on your March goal. I would continue to commute and then also start getting in one ride per week of 2hrs(usually on weekend) and increase for 3 weeks by half an hour then go back to 2-3 hrs on the fourth week(2 rest days per week). (oct)2,2.5,3,2 then (nov)3,3.5,4,2 then(dec) 4,4.5,5,2 (jan)5,5.5,6,2 etc.(get the idea). The idea is to work in increasing blocks and use that 4th week as kind of a rest week. Set this up so that your longest ride say 6-7 hrs is 2 wks out from your century. This way you will gradually increase your miles to be ready for the century and allow your body to adapt to being on the bike for extended periods. Do not be intimidated by a century!!! It is not as hard as you may think. Hope this helps. P.S. I am not a coach giving free advice but rather a regular guy who has worked with a coach and read countless books on the subject.