Preparing for a long distance ride - long term
You have more than enough time, so use it.  

Start at 25km, increase distance by 10% per week.  This would mean just six months to 280km. Key thing is start slowly, don't over do it, because of your time frame, do less not more, if you take time off (more than a couple of weeks), drop back a bit. Plot a line on a calendar - from now at 25km to May next year at 280km, this is the easiest to sustained buildup. Plot another line, From November at 25km to May. Think to yourself this is the line you must no drop under. For most people, due to 'life' getting in the way of training the reality will be in between these lines. 

Better to do 3 rides a week than one big one. Every few weeks have an easy week where you do maybe 1/2 to 2/3 of you previous week. 

The biggest obstacles will be being short of time during the week, in which case maximise you training effort, but make sure you get at least one long ride a month in. Failing to get out over winter (depending where you live) can be a problem, indoor trainer, or cross training at gym and pool will get you though winter in good shape. Still try to get out on the bike if you can.