You need to do it all, yet ease into it. I titrate it by weekly mileage. If you want to ride a 25 mile club ride, averaging 16, that's a long way from where you are now, mostly because club rides tend to go up every hill in sight. I'd say if you were riding 50 miles/week by March, you'd meet at least some of your goal. Meaning you'd get the 25 miles just fine, but maybe you'll get dropped and limp in. So be sure to get a cue sheet or know the route some other way. Getting dropped is normal. We've all been there, many times.

So get out a calendar and plan your weekly mileage between where you are now, and where you want to get to. Increase it at a steady percentage rate each week. Then space it out into your different rides. Plan it all out. But having done that, don't fixate on it. Be ready to change the program completely. You don't know what your response will be. Maybe that'll be too little and you'll want to do more, or maybe it'll make your back sore and you'll have to back off.

You'll want to ride hills at least one day a week. You don't have to actually ride all the way up them at first. Just ride until you can't and take a little rest. But hills will make you stronger and give you more endurance than anything else.