40km is about 25 miles. If you can walk 1mi, you can bike somewhere between 3 and 5 mi. And if you can bike X distance over the course of a week, you can also bike the same distance all at once. So odds are you're already *very* close to being able to bike the whole distance. Most people can walk a mile every day, even if they're in absolutely terrible shape.
You might not feel great after your first 25 mile ride, so I would try not to make the big day be the first time you do the distance. If you've done it once, you can do it again... and the 2nd time is always easier because you know what to expect. The 10th and 20th time are easier yet, tho you might not be up for that before August
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If you can, simple exercises like crunches, leg lifts, plies or squats, pushups, toe touches and the like can make things a lot more comfortable for you. I don't much like them, but even doing 5 or 10 per day can help. You don't have to do them fast or be very strong or work hard enough to break a sweat. Your core muscles are a huge help on the bike, and so is being flexible. It's very much the same principle as "ride lots"... a tiny bit at a time will bring big gains.