The Air Canada Vacations 401 Bike Challenge recommends that riders try to have at least 3,000 km in their legs prior to the start of the 401. Some riders have ridden the 401 successfully with considerably less training, others with considerably more. However, if you want to do the 3-Day Classic Ride successfully, while feeling strong throughout, 3,000 km is a good goal to strive for. Starting at the end of April over a 16-week period, that is an average of 188 km per week.
If possible, you should start riding occasionally in April, and consistently in May. Once you are consistently ride, aim to complete four rides per week. At first, these will all be relatively short rides between 30 to 60 km.
By June, you will want to start adding one long ride per week between 90 to 120 km. For your other three short rides. make them no less than 40 km. One of those short rides should incorporate some speed work, the others should be at or near tempo. Your long rides should be at a comfortable pace, but remember, you are not sightseeing.
By July, you can add a couple of things if you are up to it. Put in a couple of longer rides if you can between 120 to 150 km, and do a couple of back-to-back longer rides. These will be great for your confidence level, and will help your body get used to the idea of long days in the saddle.
It is good to ride in a group if you can. The camaraderie will keep you going and even push you to go further. Furthermore, you will begin to learn the benefits, techniques and etiquette of group riding. Check out the list of upcoming group training.
All of the targets described above should be understood within a plus/minus range appropriate to your fitness level. Secondly, it is a really good idea to do some cycling specific stretching after your rides. You can easily find these on the internet.