2 months is not a lot of time to prepare, but with a goal of finishing within the time limit that should be easily possible.

The main thing you can do is to build up your endurance with some longer weekend rides. For example:

• Week 1: 30km
• Week 2: 40km
• Week 3: 50km
• Week 4: 60km (Take an extra rest day midweek this week)
• Week 5: 75km
• Week 6: 90km
• Week 7: 75km
• Week 8: Event! (Take an extra rest day midweek this week)

Add to this by doing 2 (or 3 if you feel fresh) shorter rides midweek - starting out at 1hr and building up to 90mins. With all the rides, try not to come home exhausted, ride at a comfortable pace so you are fresh and enthusiastic for the weekend ride.

For your longer weekend rides think about how you are going to fuel yourself on event day and practice - you don't want to try out something new on the day and discover it disagrees with you. Some people like energy drink, others water and energy bars, and others real food. Generally 500-750ml/hr of fluid and ~60g of carbohydrate per hour. The longer rides will also expose any problems you may have with your contact points (Saddle, handlebars etc), and give you time to correct any problems in advance.

For the event itself, take your time and don't feel tempted to chase faster riders at the start. If you ride within your limits, take a break when you need it, and fuel well you should not only complete the event, but enjoy the experience too.

One thing I will add to the other answers is that it's now time to get a decent pair of shorts, and maybe some chamois cream.

Up to about 3 hours you can get away with anything, but for an event that might take 8--12 hours, it's worth getting some gel-padded shorts, as foam padding compresses over that sort of time. You don't have to spend a fortune, I've got some cheap ones from ebay that will do all but my toughest rides.

Try them out on your longer/later training rides just to get used to them.

Training-wise at your level the most important thing is to cycle as much (i.e. as many kilometers) as possible. If you don’t overdo it you should be able to ride every day or at least every other day (but also don’t use this as excuse to take it too easy or too short).

Other sports like running might be good for losing weight and building some endurance but in the end nothing beats bicycling if you want to be good at bicycling.

Make sure your bike fits you properly and that you wear specific road cycling clothes.

For rides longer than about one hour carbohydrates become important. During the ride you’ll need lots of easy to digest carbs. Soda, bananas, gels, chocolate bars, dried fruit (if you can digest it) etc.

It is good to make the training race-specific (in sport, intensity, duration, other aspects). I recommend concentrating on these points:

• Bike more and run less, but still keep some other sports for change and for fun. Don't forget to relax and sleep.
• Make one (or two) of your bike rides longer each week, say add 15 km each Sunday up to say 100 km week before race. It does help your body, but it also helps you MIND to get used to the longer duration efforts (both in fatigue and boredom toleration).
• Think about your nutrition during the race and try everything during your training (ideally exact brands you will take or get on race day), eating while riding (if you will do this), etc. Nutrition is BIG part of the race for races longer than two hours.
• Think about equipment, but not too much. Get the right equipment for the race (bike, clothes, spare tube pocket multi-tool, etc.), but don't spend your last penny on it, since you as an engine are the most important anyway.
• Get anti-chaffing cream or body vaseline. While not exactly necessary, it may be quite useful for longer rides.

The main point is that you will find out how most of the tips apply to you during those longer rides. For example, you may get no chaffing in 50 km, but may get unpleasant burning after 80 km.

Try everything before race in training and do not do anything new in the race. So no new shoes, shorts, food, etc. Sometimes you may be lucky and get away with it, other times it may get ugly.

I can bear almost anything for two hours, but I make everything possible to make myself comfortable (relatively) during 10 or 12-hour race.

My wife and i just blasted out a 150 km ride from Toronto to Niagara Falls on entry level aluminum/carbon bikes. I’ve been commuting 20 km per day on a bike for the last four months, but she did it without having ridden a road bike for over 5 years. If she can do it without training so can you. Just bring power bars and gatorade, along with something to change a tube.

I've just got into road bikes and want to prepare for a sportive race which is coming up in 2 months and is 150 km. My goal is just to complete the total distance within the 12 hour time limit given.

The average speed (12.5 km/h without breaks or 15 km/h with 2 hours of breaks) is doable. In varying terrain having some uphills as well, my average speed is usually 17 - 20 km/h depending on the calculation method (if I count the time standing at stoplights, 17 km/h and if not, 20 km/h).

However, the distance is quite long. You have to be in good shape and have to have excellent bike fit to be able to do that.

50 kilometers is doable without drinking or eating, but if the outdoors temperature is high, you'll want to drink something after riding 50 kilometers. No need to carry the extra weight of a full water bottle.

100 kilometers requires stopping for drinking, or alternatively carrying a water bottle. A credit card is lighter than a full water bottle, but then again credit is convertible to drinking water only in grocery stores.

150 kilometers probably requires stopping for eating as well.

As of now I ride (16 km in 45 min)/day for 5-6 days a week and have decent fitness level but not too great . Apart from this I don't really play any sport but I jog (8 km in 45 min)/day everyday.

You need to start increasing your daily amounts. Sixteen kilometers is not much. I had a long break from cycling, was in really bad shape and since starting riding, it took two weeks to reach 45 kilometers in a day. That was without any electric assist (today I have an electric bike as well). Getting to 50 kilometers per day is easy. Getting to 100 kilometers per day is hard as you'll be exhausted. Getting to 150 kilometers per day is very, very hard indeed (I have never ridden 150 kilometers in a single day, bit over 100 kilometers has been my maximum). The problems in your bicycle fit won't be noticeable if you keep your daily amounts below 50 kilometers.

The terrain for the race and where I am going to practice is completely flat. I can devote 1-1.5 hours a day on the weekdays and much more on the weekends for training.

The problem is that 1 - 1.5 hours won't "train" you enough for a long ride. Prefer to do the preparations on weekends when you have more time. Anyone in adequate shape can ride 1 - 1.5 hours, and repeating the exercise ten or hundred times won't change much.

However, if the terrain is completely flat, you can very easily adjust the level of effort by varying your speed. On flat terrain, the power need is proportional to velocity cubed due to peculiar properties of air resistance. So, 20% reduction in velocity is equal to 48.8% reduction in effort. Thus, you can very easily halve your effort by reducing the velocity slightly. This means flatlands riding can be as easy as you want to make it.

On uphills the situation is different. The power need is proportional to velocity. A 20% reduction in velocity is equal to 20% reduction in effort. To halve your effort, you need to halve your velocity, which may be too large drop -- for example, it might be hard to stay upright if you halve the uphill climbing velocity.