Richboot, I think people are really underestimating what an endeavor this is. Congratulations for even considering this! I mean, 700 miles in a week is ambitious, but to add the English channel by bike, AWESOME!
Okay, seriously, you'll need significant time on the bike to make this happen. When I'm trained 100 miles by bike equates to 13-15 miles running, the bike trip takes at least twice as long but the impact on my body is roughly the same. Considering that you need to evaluate yourself and find out where you're starting from. 5k 3 days/week would not be anywhere near enough to get me into shape to run a half marathon 7 days in a row. If I were running this Id be doing 2 weekdays with 10ks at a hard aerobic pace, somewhere sub-50 minutes. Another weekday would be speed intervals fartleks or a 5k at my best pace, sub 20 minutes. And 6-8 hours of LSD divided up between the two weekend days.
Cycling volumes tend to be greater than running volumes, by about twice as much. So to train for the event you're planning, you will need a fair amount of saddle time. Probably the most important adaptations you're aiming for are being able to sit on the bike basically forever. This takes time. But, you also need to build up your speed. Granted you aren't racing but I hope riding in a group, being well below your personal 'redline' for the duration makes the week all about pacing, nutrition, and enjoying yourself. If you're struggling to keep up with the group you will suffer.
I'd suggest fore going any significant running and do all your cardio on the bike, I'd also say skip the weights and do a ride on those days as well but some people are reluctant to give up their weight lifting so Id just say don't aim to build muscle or make any big strength gains and at the most maintain. Also, don't expect any weight training to benefit your cycling speed (for this event, I really don't want to start the argument of weight training for cycling performance.) Suffice to say that the nature of the event and your background suggest that the best use of time is building endurance on the bike.
I'd start building up miles riding a steady pace for a few weeks or months, but once you're accustomed to riding a fair amount start adding intensity and volume. 2 days of hard tempo rides lasting 2-3 hours each, a day of threshold work 2x20 or even 30 minute intervals at LT, and 2 weekend days of several hours at a slow very aerobic pace should be your goal for the last couple of months leading up to next August. As I said before hopefully you're riding with a group, if you can find a group ride in your area to join for at least one ride a week it will pay off to be comfortable rubbing hips elbows and wheels, not to mention learning to ride in the draft of others. A lot of clubs have regular slow to moderate paced rides on weekends and as your fitness improves you can look for your area's tues/thurs "world championship" ride for one of those 'tempo' days.
I usually aim for 15 hours a week of riding at a bit higher intensities than the above suggestion, I am in good racing shape. I time trial very well and climb adequately, I would NOT jump into the event you're planning without reservation.
make sure you're properly fitted on the bicycle!