What is your route? If you are planning on doing in a couple of years then there isn't much point trying to get touring-fit now. You should just eat right and do exercise, whether that includes a bike or not is up to you. I've seen all sizes of people do it, but you'll probably enjoy it more the less body weight you are carrying.

I think you should start to prepare for the tour physically maybe 3-4 months out, as others have said, build up your ride distance gradually until you feel comfortable riding 2-3 hours at a time, once you can do that, you can do a coast to coast no sweat.

If you are looking @ 70 miles a day average, that is quite a high average for a coast to coast ride I reakon. are you planning on treating it like a race, or is the 7 weeks an imposed limit due to work/other commitments ? I think 50-60 is a good daily avearge, depending on your route, you will have higher days and lower days, but you must leave yourself time for days off the bike to explore and recharge those legs. On my C2C I took over half of my rest days within the first 3 weeks, the further I got into it , the fitter I got and more tour-hardy my legs got, so my advice would be start out slow and build up to that average.