We all start from different fitness levels and some of us have more time to commit.

If you take 2 weeks off, it can take you 6 weeks to return to the same level of overall fitness. Be consistent and build slowly. Not knowing your background and a limited information about the type of riding that you have done over those 3 months; thus, we can only give general suggestions. To do a 200K ride, your currently weekly mileage needs to rise well beyond the current 75Km to perhaps 200Km per week but the most important ride is increasing the one long ride per week. You want to slowly build this long ride up as Macha indicated. You should not push hard on long rides. Your pace on long rides (currently your 50KM ride) should be moderate. Not slow but not fast.....in cycling Geekspeak....low Zone 2. You should be able to chat several sentences at a time. I would venture a guess that your anaerobic energy systems are getting plenty of workouts from the traffic lights.

I would not do more than one really hard Stava type ride per week initially but you do need one hard ride in addition to the long ride. Commuting should round it out for a while. You will get stronger. Just build slow and consistent.